The genetic diversity of Chinese hamster cell mutants exhibiting hypersensitivity to the bifunctional alkylating agent mitomycin C has been examined. The eight mutants irs3, VH4, UV1, MC5, MMC1, MMC3, MMC4 and MMS2, are between 4- and 30-fold more sensitive to mitomycin C than their respective wildtype parental lines. A number of the mutants show phenotypic similarities to cultured cells from the human cancer-prone syndrome Fanconi's anaemia. Hybrids were formed between pairs of mutants using the thioguanine/ouabain resistant (TOR) hybridization and hypoxanthine/aminopterin/thymidine (HAT)/ouabain selection system and the mitomycin C response of pooled populations of hybrids assessed by constructing survival curves. In every case, hybrids formed between pairs of mutants exhibited a mitomycin C response indistinguishable from that of wildtype cells, indicating complemlenation. Therefore, the eight mutant lines examined represent eight distinct complementation groups for mitomycin C-hypersensitivity. The results are in contrast to the complementation analysis of UV-sensitive Chinese hamster cell mutants and indicate that the response of mammalian cells to mitomycin C-induced DNA damage is complex and involves a large number of genes.