It was found that the water extract of grass-wrack pondweed (Potamogeton oxyphylus Miquel) had inhibitory activity for reverse mutations induced by benzo[a]pyrene, 2-aminoanthracene and 2-nitrofluorene with Salmonella typhimurium TA100 and TA98. The active substance was heat-resistant, and the molecular weight was above 300 000. The substance acted as a desmutagen rather than an antimutagen or inhibitor of metabolic activation.