RECENTLY, Comb and Roseman1 observed that N-acetyl-d-mannosamine and not N-acetyl-d-glucosamine was a substrate for the enzyme synthesizing N-acetylneuraminic acid (nanaldolase) isolated from Clostridium perfringens. These results seem to be in conflict with the synthesis of N-acetylneuramic acid from N-acetyl-d-glucosamine as reported by Corn-forth, Firth and Gottschalk2. The observations of Comb and Roseman suggest that N-acetyl-d-mannosamine should be the starting product for the synthesis of N-acetylneuraminic acid.