SUMMARYThe cross-resistance patterns are described for 96 mutants selected in one step from Escherichia coli strain S for resistance to Mitomycin C. The test agents used were ultraviolet radiation, seven radiomimetic and two non-radiomimetic compounds. Seven different types of mutants could be selected in one step from the parent. Five of these were radioresistant; two were chemoresistant. Of the radioresistant types two were identical with types previously isolated using other radiomimetic agents for selection; three of the types were new. One of the new types was reactivated by plating medium following ultraviolet irradiation differently from all other radioresistant mutants of E. coli S. One of the chemoresistant types was resistant only to Mitomycin C; the other displayed a low degree of cross-resistance to nitronitrosoguanidines and to penicillin. Also described are the cross-resistance patterns of mutants selected in five consecutive steps for increasing resistance to Mitomycin C. Beginning with a first step radioresistant mutant it was possible to select four additional steps, up to 325-fold, in resistance to Mitomycin C., One of these steps appeared to be a shift from one radioresistant type to another.
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