The mutagenicities of 7 pyrrolizidine alkaloids to Salmonella typhimurium TA100 were demonstrated by a modified Ames's method. The pyrrolizidine alkaloids found to be mutagenic were clivorine, fukinotoxin, heliotrine, lasiocarpine, ligularidine, LX201 and senkirkine. Pre-incubation of these alkaloids with S9 mix and bacteria in a liquid medium was essential for demonstration of their mutagenicities.