As a consequence of the recent high precision determination of the absolute configuration of mitomycin C (IC) as C1 (SI, C2 (S), Cg (S), Cg,(R) by X-ray crystallographic analysis, l there now exists uncertainty about the stereochemical and perhaps even the biosynthetic relationships among some of the naturally occurring mitomycins. 2 This situation prompts us to report on studies which show that mitomycin A (Ia) and mitomycin B (11) have the same