diff --git a/gcode_parse.c b/gcode_parse.c
index 6a1e771..bb5ca6e 100644
--- a/gcode_parse.c
+++ b/gcode_parse.c
@@ -18,6 +18,7 @@
   #include "simulator.h"
 #endif
 
+
 /// current or previous gcode word
 /// for working out what to do with data just received
 uint8_t last_field = 0;
@@ -31,6 +32,12 @@ decfloat BSS read_digit;
 /// this is where we store all the data for the current command before we work out what to do with it
 GCODE_COMMAND BSS next_target;
 
+#ifdef SD
+  #define STR_BUF_LEN 13
+  char gcode_str_buf[STR_BUF_LEN];
+  static uint8_t str_buf_ptr = 0;
+#endif
+
 /*
 	decfloat_to_int() is the weakest subject to variable overflow. For evaluation, we assume a build room of +-1000 mm and STEPS_PER_MM_x between 1.000 and 4096. Accordingly for metric units:
 
@@ -117,6 +124,13 @@ void gcode_parse_char(uint8_t c) {
 					break;
 				case 'M':
 					next_target.M = read_digit.mantissa;
+          #ifdef SD
+            if (next_target.M == 23) {
+              // SD card command with a filename.
+              next_target.read_string = 1;  // Reset by string handler or EOL.
+              str_buf_ptr = 0;
+            }
+          #endif
 					if (DEBUG_ECHO && (debug_flags & DEBUG_ECHO))
 						serwrite_uint8(next_target.M);
 					break;
@@ -202,8 +216,11 @@ void gcode_parse_char(uint8_t c) {
 		}
 	}
 
-	// skip comments
-	if (next_target.seen_semi_comment == 0 && next_target.seen_parens_comment == 0) {
+  // Skip comments and strings.
+  if (next_target.seen_semi_comment == 0 &&
+      next_target.seen_parens_comment == 0 &&
+      next_target.read_string == 0
+     ) {
 		// new field?
 		if ((c >= 'A' && c <= 'Z') || c == '*') {
 			last_field = c;
@@ -371,7 +388,8 @@ void gcode_parse_char(uint8_t c) {
 			next_target.seen_P = next_target.seen_T = next_target.seen_N = \
       next_target.seen_G = next_target.seen_M = next_target.seen_checksum = \
       next_target.seen_semi_comment = next_target.seen_parens_comment = \
-      next_target.checksum_read = next_target.checksum_calculated = 0;
+      next_target.read_string = next_target.checksum_read = \
+      next_target.checksum_calculated = 0;
 		// last_field and read_digit are reset above already
 
 		if (next_target.option_all_relative) {
@@ -381,6 +399,20 @@ void gcode_parse_char(uint8_t c) {
       next_target.target.axis[E] = 0;
 		}
 	}
+
+  #ifdef SD
+  // Handle string reading. After checking for EOL.
+  if (next_target.read_string) {
+    if (c == ' ' || c == '*') {
+      next_target.read_string = 0;
+    }
+    else if (str_buf_ptr < STR_BUF_LEN) {
+      gcode_str_buf[str_buf_ptr] = c;
+      str_buf_ptr++;
+      gcode_str_buf[str_buf_ptr] = '\0';
+    }
+  }
+  #endif /* SD */
 }
 
 /***************************************************************************\
diff --git a/gcode_parse.h b/gcode_parse.h
index d15716d..5b19b90 100644
--- a/gcode_parse.h
+++ b/gcode_parse.h
@@ -4,6 +4,7 @@
 #include	<stdint.h>
 
 #include	"dda.h"
+#include "sd.h"
 
 // wether to insist on N line numbers
 // if not defined, N's are completely ignored
@@ -38,6 +39,7 @@ typedef struct {
 		uint8_t					seen_checksum				:1; ///< seen a checksum?
 		uint8_t					seen_semi_comment		:1; ///< seen a semicolon?
 		uint8_t					seen_parens_comment	:1; ///< seen an open parenthesis
+    uint8_t         read_string         :1; ///< Currently reading a string.
 		uint8_t					option_all_relative	:1; ///< relative or absolute coordinates?
 		uint8_t					option_e_relative		:1; ///< same for e axis (M82/M83)
 		uint8_t					option_inches				:1; ///< inches or millimeters?
@@ -62,6 +64,11 @@ typedef struct {
 /// the command being processed
 extern GCODE_COMMAND next_target;
 
+#ifdef SD
+  /// For storing incoming strings. Currently the only use is SD card filename.
+  extern char gcode_str_buf[];
+#endif
+
 void gcode_init(void);
 
 /// accept the next character and process it
diff --git a/gcode_process.c b/gcode_process.c
index 3a29400..25910cf 100644
--- a/gcode_process.c
+++ b/gcode_process.c
@@ -398,6 +398,14 @@ void process_gcode_command() {
         //? inserted.
         sd_unmount();
         break;
+
+      case 23:
+        //? --- M23: select file. ---
+        //?
+        //? This opens a file for reading. This file is valid up to M22 or up
+        //? to the next M23.
+        sd_open(gcode_str_buf);
+        break;
       #endif /* SD */
 
 			case 82:
diff --git a/sd.c b/sd.c
index 9339a30..6fe93d1 100644
--- a/sd.c
+++ b/sd.c
@@ -69,4 +69,23 @@ void sd_list(const char* path) {
   }
 }
 
+/** Open a file for reading.
+
+  \param filename Name of the file to open and to read G-code from.
+
+  Before too long this will cause the printer to read G-code from this file
+  until done or until stopped by G-code coming in over the serial line.
+*/
+void sd_open(const char* filename) {
+  result = pf_open(filename);
+  if (result == FR_OK) {
+    // To demonstrate this works, open the file and show its size.
+    // Actually, the file name isn't stored, so we can't read it back :-)
+    sersendf_P(PSTR("Successfully opened file with %lu bytes."), sdfile.fsize);
+  }
+  else {
+    sersendf_P(PSTR("E: failed to open file. (%su)"), result);
+  }
+}
+
 #endif /* SD */
diff --git a/sd.h b/sd.h
index df4a39b..7252add 100644
--- a/sd.h
+++ b/sd.h
@@ -23,6 +23,8 @@ void sd_unmount(void);
 
 void sd_list(const char* path);
 
+void sd_open(const char* filename);
+
 #endif /* SD_CARD_SELECT_PIN */
 
 #endif /* _SD_H */