AbstractThe genetic activity of PR toxin, a mycotoxin from Penicillium roqueforti, was studied in Saccharomyces cerevisiae and Neurospora crassa. The results show that PR toxin, without enzymic activation, causes gene conversion in S. cerevisiae strains D4 and D7, reverse mutation in S. cerevisiae strain D7 and N. crassa strain N24, and mitotic crossing‐over in S. cerevisiae strain D7, In the log phase cells of S. cerevisiae the effects are more pronounced at alkaline than at acid pH. The active site responsible for the genetic activity and toxicity is known. The carbonyl groups appear to play an important role in the biologic activity of this molecul