The effects of genistein on testicular cells, TM3, TM4, and GC-1 spg, were studied in vitro. First, each cell line was cultured with pre-determined concentrations of genistein for a maximum of 72 h to assess the effects of genistein on in vitro growth of the test cells. A second series of experiments were performed to determine the degree of genistein-induced apoptosis in these cells, using Apop-Tag kit reagents, to detect apoptotic cells in situ by specific end labeling, and detection of DNA fragments produced by the apoptotic process. The results obtained indicate that: i) genistein inhibits the growth and proliferation of testicular cells; ii) growth inhibition and proliferation is dose- and exposure-time dependent; iii) there is significant difference in sensitivity of the different testicular cells to genistein; iv) genistein induces apoptosis in testicular cells in a concentration-dependent manner. Genistein-induced apoptosis identifies genistein as a potential diagnostic and therapeutic tool in testicular pathophysiological research.