The relative mutagenic and lethal effects of 2-methoxy-6-chloro-9-(3- (ethyl-2-chloroethyl)aminopropylamino)acridine 2HCl (146598) (ICR- 170) were studied in Neurospora (N)-crassa. Conidial suspensions of adenine-3 presumptive base pair addition or deletion mutants and adenine-3 base pair substitution mutants of N-crassa were prebubbled with nitrogen or oxygen before ICR-170 was added, were bubbled with nitrogen or oxygen after ICR-170 treatment, or were incubated with ICR-170 in the presence of nitrogen or oxygen. Samples were removed after treatment, and mutation and killing activities were determined. The percent survival was significantly less when conidia were exposed to nitrogen during the ICR-170 treatment than when they were exposed to oxygen during treatment, but survival and reversion frequencies were the same when oxygen or nitrogen was given before or after ICR-170 exposure. The nitrogen effect was the same in all strains tested. The high killing activity of ICR-170 induced mutation rate under nitrogen was attributed to enhancement of the mutagenic activity of ICR-170 rather than to selective killing. The author concludes that the increased toxicity and mutagenicity of ICR-170 when given with nitrogen is due to inhibition of conidial respiration in an anoxic environment.