The isoflavonoid phytoalexins, kievitone and phaseollin [from French bean], were selectively toxic to gram-positive bacteria. In a standard paper-disk bioassay, 10-50 .mu.g kievitone or phaseollin inhibited the growth of all 7 gram-positive, but none of the 8 gram-negative, bacteria tested. Phaseollidin and phaseollinisoflavan also possessed such selective toxicity to gram-positive bacteria. Further assays showed that even 2 .mu.g kievitone (0.56 .times. 10-8 mol), which were the most toxic of the compounds examined, inhibited the growth of Corynebacterium fascians, Bacillus subtilis and Micrococcus luteus.