The mutagenic effects of metronidazole (443481) and niridazole (61574) were studied in Neurospora-crassa (N-crassa). Resting conidia from a genetically marked heterokaryon of N-crassa in the resting stage and vegetative N-crassa cells were tested for mutagenicity and mutagenic specificity by the adenine-3 (ad-3) test system, using various concentrations of metronidazole and niridazole. In resting conidia, metronidazole and niridazole did not significantly increase mutagenicity or decrease survival. In two experiments niridazole decreased survival from 100 to 86 and 81 percent in vegetative cultures of N-crassa, and the ad-3 frequency of mutation increased to 25.4 and 22.4 mutants per 1,000,000 survivors compared to 0 per 1,000,000 in untreated cultures. In vegetative cells, metronidazole was both mutagenic and lethal, causing a dose related decrease in survival of cultured cells. Ad- 3A and ad-3B mutants occurred in 20 and 80 percent of niridazole induced and 43 and 57 percent of metronidazole induced mutants, respectively. The authors conclude that metronidazole and niridazole are not toxic or mutagenic to resting conidia of N-crassa but are toxic and mutagenic to cells in the vegetative stage.