Gossypol, a yellow pigment from the cotton plant (Gossypium) was found to have anti-tumor cell effects against several tumor cell lines grown in tissue culture. Most sensitive to the drug were melanoma and colon carcinoma cells. After 24 h of treatment with 10 .mu.M gossypol, over 90% of these cells were killed. A rough correlation existed between very rapidly growing cells and their sensitivity to the drug. For example, slow growing cells, such as normal embryonic lung fibroblasts and mammary adenocarcinoma, were least sensitive, requiring > 30 .mu.M gossypol in order to kill 90% of cells after 48 h of treatment. These results indicate that gossypol has differential cytotoxic effects against certain tumor cell types, such as melanoma and colon carcinoma, and suggest that it might be of potential therapeutic value. The drug appears to act as a metabolic poison rather than as an agent interfering with DNA synthesis at the concentrations used in the study. Cells sensitive to the drug contain the more cathodic forms of lactate dehydrogenase, lactate dehydrogenases I to III.