The polyanionic compound suramin can inhibit the proliferation of cells of various origin, including from breast cancer. We have studied the effects of suramin on cell cycle kinetics and distribution of MCF-7 human breast cancer cells in vitro. It was found that both under serum-containing and serum-free culture conditions, and in the absence or presence of oestradiol or insulin-like growth factor-1, prolonged exposure (> or = 48 h) to suramin caused an accumulation of surviving cells in the G2/M-phase of the cell cycle. At a concentration of more than 100 micrograms ml-1 suramin significantly inhibited cell proliferation. The observed effects of suramin on breast cancer cells in vitro, i.e. antiproliferative effects and accumulation of cells in the G2/M-phase of the cell cycle, may have beneficial consequences in the application of treatment strategies based on a combination of suramin with cell cycle specific drugs or radiation therapy.