diff --git a/src/s/irix6-5.h b/src/s/irix6-5.h
index c1cc42a8c0ec48ebddcd608f108cdade9bad31ca..9df3e0a2ef6c0ff096e54b3b32e7ebb4a804becb 100644
--- a/src/s/irix6-5.h
+++ b/src/s/irix6-5.h
@@ -33,5 +33,9 @@
 
 #undef TIOCSIGSEND		/* defined in usg5-4.h */
 
+/* Tested on Irix 6.5.  SCM worked on earlier versions.  */
+#define GC_SETJMP_WORKS 1
+#define GC_MARK_STACK GC_MAKE_GCPROS_NOOPS
+
 /* arch-tag: d7ad9ec2-54ad-4b2f-adf2-0070c5c63e83
    (do not change this comment) */