A method using murine P-388 leukemia or human HT-29 colon carcinoma cells was developed for the bioautography of potential antitumor agents. Of 18 cancer chemotherapeutic drugs and natural products tested, all were detected by toxicity at 0.01 or 1.0 micrograms with P-388 cells, and 11 of the 18 were detected at 10 micrograms or less with HT-29 cells. Bioautography of a crude extract of Pseudoplexaura wagenaari and subsequent purification yielded the known compound crassin acetate. With modification, the assay detected specifically toxic DNA-binding agents.