Mycobacteria stimulate the lymphoreticuloendothelial system. We confirmed that BCG exerts an antitumor effect on the B16-F1 melanoma cell line in C57BL/6 mice and attempted to develop the B16-F1 melanoma as a model of activity for PS1, a polysaccharide antineoplastic immunomodulator isolated from the TiceŽ substrain of BCG vaccine. However, PS1 was ineffective in preventing B16-F1 primary tumor growth, suggesting that although BCG was effective against melanoma, its activity was not mediated by PS1 immunostimulation. Both BCG and a filtrate of BCG vaccine exerted an inhibitory effect on the growth of B16-F1 cells in vitro, probably by inhibiting their adherence to a solid substrate. This suggests the presence of an anti-adherence factor, produced by BCG organisms, which contributes to the antineoplastic activity. Introduction Mycobacteria can activate macrophages, which are cytotoxic for a wide variety of malignant cells and appear to play a role in immunity to cancer. The BCG components responsible for its immune adjuvant activity, and their mechanism of action, remain unknown. High-dose BCG tumor-inhibitory activity in a mouse sarcoma model appears to be mediated by lymphocytes. However, there are problems in the utilization of BCG as an antitumor agent, since it consists of viable organisms. A mixture of polysaccharide immunomodulators, identified as PS1, has been extracted from BCG vaccine, substrain TiceŽ, with potential for immunotherapy of some cancers without producing the adverse effects associated with the administration of live organisms. PS1 was shown to exert antitumor activity in vivo using the S180 murine sarcoma model, but was inactive against tumor cells in vitro.