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‐TagR 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.