diff --git a/erlang/apps/as/src/parse.erl b/erlang/apps/as/src/parse.erl
index a7420f0..a7d2802 100644
--- a/erlang/apps/as/src/parse.erl
+++ b/erlang/apps/as/src/parse.erl
@@ -192,7 +192,7 @@ insn_ea_index(ScanState, Location, Name, AccOrDev, At, Displacement) ->
 
 make_insn(Location, Name, AccOrDev, At, Displacement, Index) ->
   Models = ?PDP10_KL10_271, % FIXME: make dynamic
-  case pdp10_opcodes:insn_from_name(Models, Name, AccOrDev =/= false) of
+  case insn_from_name(Models, Name, AccOrDev =/= false) of
     false -> badinsn(Location, "invalid mnemonic ~s", Name);
     #pdp10_insn_desc{ high13 = High13
                     , format = Format
@@ -218,6 +218,15 @@ make_insn(Location, Name, AccOrDev, At, Displacement, Index) ->
       end
   end.
 
+insn_from_name(Models, Name, HaveA) ->
+  case pdp10_opcodes:insn_from_name(Models, Name, HaveA) of
+    false ->
+      %% To simplify porting code written for MACRO-10 we accept
+      %% mnemonics written in upper-case.
+      pdp10_opcodes:insn_from_name(Models, string:lowercase(Name), HaveA);
+    Insn -> Insn
+  end.
+
 make_high13(Location, Name, AccOrDev, High13, Format) ->
   case {Format, AccOrDev} of
     {?PDP10_INSN_A_OPCODE, false} ->