The genetic effects of PR-toxin (56299004) were studied in Saccharomyces-cerevisiae and Neurospora-crassa. The D4 strain of Saccharomyces-cerevisiae was used for estimating the mitotic gene conversion at the ade 2 and trp 5 loci, and the D7 strain was used for studying mitotic gene conversion at the trp locus, point reverse mutation at the ilv locus, and mitotic crossing over at the ade locus. The N24 and N23 adenine dependent mutant strains of Neurospora-crassa were used. For each 0.2 milliliter (ml) of the cell or conidial suspension, 4.6ml of 0.1 molar phosphate buffer at various pH and different concentrations of the test substance were dissolved in 0.2ml of dimethyl-sulfoxide and incubated for either 1 or 2 hours. Survival was determined 2 to 3 days after incubation. Gene conversion, reversion an mitotic crossing over were scored 7 to 10 days after plating. PR-toxin was highly toxic, and the extent of toxicity varied with the pH of the treatment, its duration, and the population and phase of the microorganism being treated. A survival level above 5 percent, PR-toxin produced mitotic gene conversion in both strains of Saccharomyces-cerevisiae. PR-toxin also produced reverse mutations in the D7 and N24 strains as well as mitotic crossing over in the D7 strain. The authors conclude that PR-toxin is a direct acting mutagen and has a toxic effect without exogenous metabolic activation.