The antiproliferative activity of lonidamine, alone or in combination with the antiestrogen tamoxifen, was studied on estrogen receptor-positive and estrogen receptor-negative human breast cancer cell lines. Lonidamine by itself induced an appreciable cytotoxic effect on all five cell lines, with 50% inhibitory concentrations (IC50) ranging from 19.5 to 54 mu M. The combination of lonidamine and tamoxifen, simultaneously administered or in sequence, provided additive effects on the estrogen receptor-negative MCF/DX cell line and a sub-additive interaction in the estrogen receptor-positive MCF7 cells. The negative interference between the two drugs could be ascribed to the marked reduction induced by lonidamine in the expression of estrogen receptor in the MCF7 cells.