EtOH extracts of the seeds of Euphorbia lagascae inhibited crown gall tumors on potato discs, were active against 3PS (in vivo) and 9PS (in vitro) mouse leukemia, and were toxic to brine shrimp. Using the potato disc and brine shrimp bioassays to monitor fractionation, piceatannol (3,4,3',5'- tetrahydroxystilbene ) was isolated as the 3PS active principle. Aesculetin (6,7-dihydroxycoumarin) was isolated in large quantity, was weakly cytotoxic in 9KB , but was inactive in the other bioassay systems. Sucrose was also isolated. A combination of the simple crown gall and brine shrimp bioassays can thus be used both to detect and to isolate plant antitumor substances, minimizing the need for extensive antitumor testing in vivo.