Tansy ragwort (Senecio jacobaea ) was evaluated for animal and human health hazard using the Salmonella/mammalian-microsome mutagenicity test. An acetone extract of tansy ragwort (TR) produced a negative mutagenic response for bacterial tester strains TA1535 and TA100 and a toxic response in tester strains TA1537 and TA98. Assay of this extract in the presence of mammalian liver microsomes (S-9) resulted in positive mutagenic responses in tester strains TA1535, TA1537, TA98 and TA100. Species differences were evaluated by use of liver microsome preparations from induced rat and uninduced sheep, beef, hamster, trout and rat. Only a slight species difference was demonstrated. A mixture of pyrrolizidine alkaloids (PA), extracted from TR flowers, produced a negative response in tester strains TA1535, TA1537, TA98 and TA100. A negative response was also demonstrated when the TR flower PA mixture was assayed with the Salmonella tester strains and induced rat liver microsomes ( IRLM ). A mixture of PA extracted from Senecio longilobus also produced a negative response. The major PA present in TR, jacobine , produced a negative response without and with IRLM exposure in tester strains TA1535, TA1537, TA98 and TA100. Another similar PA, monocrotaline, found in various Crotalaria species also gave a negative response. Milk from TR-fed goats was evaluated for mutagenic response. Milk from goats not receiving TR and from goats receiving TR at a level of 1% of their body weight/day via rumen cannula produced a negative response without liver microsomes present. Milk from TR-fed goats, however, yielded both negative and marginally positive responses for different combinations of tester strains and liver microsome preparations.