Eighteen pyrrolizidine alkaloids and several derivatives have been examined for tumor‐inhibitory properties. Of these, 10 compounds show significant activity against one or more test tumors. Heliotrine, lasiocarpine, monocrotaline, spectabiline, and senecionine are highly active against the Walker 256 (intramuscular) system. The activity pattern is consistent with the tumor‐inhibitory action being associated with the allylic ester function which imparts alkylating ability and is also responsible for hepatotoxicity. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company