Allomelanins, which belong to the melanins produced by higher plants and fungi, were studied with respect to their growth suppressive effects on cultured HCT-15 cells derived from human intestinal carcinoma and Meth/A cells derived from a Balb/C mouse lymphoma. Allomelanins were extracted from black soy beans and black sesame seeds with 0.2% NaOH in water. Proteins bonded to the allomelanins were removed by boiling the extracts for 20 hours in 30% NaOH. Although native allomelanins conjugated with protein did not suppress the growth of the cells, protein-free allomelanins did suppress it. That is, 50% suppression of HCT- 15 cell growth was found at a concentration between 100 and 200 micrograms/ml of the melanin prepared from the black soy beans, and between 25 and 100 micrograms/ml of that from the sesame seeds. For Meth/A cells, 50% suppression by allomelanin from the black beans was approximately 100 micrograms/ml; and that by the allomelanin preparation from sesame seeds, between 25 and 100 micrograms/ml. Thus, protein-free allomelanins seemingly suppress cultured mammalian and animal tumor cells.