diff --git a/mps/manual/build.txt b/mps/manual/build.txt
index 4929c9b7b1f..3c15d654e1a 100644
--- a/mps/manual/build.txt
+++ b/mps/manual/build.txt
@@ -34,7 +34,7 @@ This will build a "hot" variety (for production) object file for use
 with "mps.h".  You can greatly improve performance by allowing global
 optimization, for example:
 
-    cc -O3 -c mps.c       (Unix/Mac OS)
+    cc -O2 -c mps.c       (Unix/Mac OS)
     cl /O2 /c mps.c       (Windows)
 
 
@@ -59,7 +59,7 @@ myformat.c, then you could make a file mymps.c containing
 
 then
 
-    cc -O3 -c mymps.c     (Unix/Mac OS)
+    cc -O2 -c mymps.c     (Unix/Mac OS)
     cl /O2 /c mymps.c     (Windows)
 
 This will get your format code inlined with the MPS garbage collector.