Mitomycin c in the presence of NADPH and brewers' yeast NADPH: (acceptor) oxidoreductase (Old Yellow enzyme, EC 1.6.99.1) is transformed, at pH 8.0 and with anaerobicity, to two major mitosene products (the cis- and trans-1-hydroxy-2,7-diaminomitosenes; respective yields of 45 and 30%). These arise by covalent trapping by solvent of a quinone methide intermediate, obtained by rearrangement of the mitomycin c hydroquinone. At lower pH (6.5), the major product of this reaction is 2,7-diaminomitosene, which arises by covalent trapping of the quinone methide by H+. In the former instance the quinone methide acts as an electrophile and in the latter as a nucleophile. A detailed kinetic analysis indicates that the role of the NADPH and Old Yellow enzyme is to initiate an autocatalytic reaction, propagated by mitomycin c reduction by the mitosene hydroquinones (arising by the electrophilic pathway). The evidence supporting this conclusion is the formation of oxidized mitosene products, under the rigorously anaerobic reaction circumstance, the nonstoichiometric participation of NADPH, a dependence of the velocity on the total mitomycin c concentration, the pH dependence of the reaction, and the accurate simulation of the overall kinetic course with a mathematical model of the autocatalytic pathway. These observations indicate that the autocatalytic pathway may be dominant during in vitro mitomycin c anaerobic reductive activation by other reducing agents and that (as with anthracycline reductive activation) oxidation of the mitosene hydroquinone obtained from nucleophile addition to the quinone methide may be a necessary event for the formation of stable covalent adducts in vivo.