The molecular basis of bleomycin (BLM)-induced mutations in the absence and presence of inhibitors of DNA repair was investigated in V79 cells with Southern hybridization techniques. 43% of the BLM-induced thioguanine-resistant mutants suffer from large alterations of hprt DNA sequences. To understand the role of DNA repair in the process of mutagenesis, the effect of inhibitors of DNA repair on the frequency and types of BLM-induced mutations was tested. The inhibitors used were arabinofuranosyl cytosine (araC), didesoxythymidine (ddThd) and 3-aminobenzamide (3AB), which inhibit different steps of excision repair. Only 3AB caused a comutagenic effect. The increased mutation frequency was mainly due to additionally induced gene deletions. In the presence of 3AB, 70% of the HPRT-deficient mutants revealed partial or total deletions of the hprt coding sequences. Thus, it could be shown that BLM induces a broad range of types of mutation and that inhibited repair of BLM-induced DNA damage leads to specific types of mutations.