Suramin is the first putative growth factor inhibitor in clinical trial that has demonstrated antitumor activity. Bivariate flow cytometric analysis of the effects of suramin on the cell cycle was performed on DU-145 prostate carcinoma cell line. The effects of suramin on the expression of proliferation associated antigens (p145, p120, PCNA, Ki-67, and cyclin A) were also studied. At concentrations of 50-500 micrograms/ml, suramin arrested cells in G1 and decreased S-phase moderately. The expression of p145, p120, PCNA, Ki-67 and cyclin A in these cells was reduced. The data suggest that suramin not only inhibits proliferation, but also reduced the expression of proliferation associated antigens. The expression of these antigens may be considered a means to monitor suramin treatment in vivo and in vitro.