In this study we provide evidence that tamoxifen, the widely used breast cancer drug, is a potent antagonist of glycolipid metabolism. When added to the medium of cultured multidrug resistant (MDR) KB-V-1 carcinoma cells, tamoxifen, at 5.0 microM, drastically lowered the levels of glucosylceramide (glc-cer), as evidenced by a reduction in glc-cer mass. In a similar fashion, in cultured human melanoma cells grown with [3H]galactose, tamoxifen inhibited formation of glc-cer by 44%, and retarded lactosylceramide and ganglioside formation by 50 and 35%, respectively. When glc-cer synthase of melanoma was assayed in cell-free incubations, the inclusion of tamoxifen, at a 1:10 molar ratio with ceramide, inhibited glc-cer synthesis by 50%. These results clearly reveal a new action of tamoxifen and thereby pose intriguing questions regarding mechanisms of action in the realm of estrogen receptor-independent modalities, including effects on MDR.