Bleomycin is active as mutagen in Salmonella typhimurium TA102, the strain which acts preferentially at A-T base pairs. Bleomycin has been postulated to produce toxicity through the generation of superoxide anion. The mechanism of mutagenic action can be the effect of free radical action. If bleomycin is a generator of superoxide radical the level of superoxide dismutase (SOD) can affect the mutagenic activity of this drug.As an inhibitor of SOD activity diethyldithiocarbamate was used. The inhibition of SOD activity after preincubation with diethyldithiocarbamate in concentrations 0.1 to 20 mM was observed.The number of revertants his + induced by bleomycin in Salmonella typhimurium TA102 decreased after preincubation with 20 mM SOD inhibitor but lower concentrations of inhibitor did not change the revertants number. However, bleomycin treatment after preincubation with diethyldithiocarbamate inhibits growth of Salmonella typhimurium TA102. Decreased growth suggests that the number of his + revertants per living cells is not the same but higher than in control. Results of our experiments show that the level of SOD activity plays a role in mutagenesis by bleomycin in Salmonella typhimurium TA102. These data may suggest the involvement of superoxide anion in bleomycin mutagenesis.