Epidemiological studies report that regular physical activity can reduce the risk for prostate cancer, the most common solid-tumor cancer in US men. Regular exercise alters the serum IGF axis in vivo and reduces cell proliferation while increasing apoptosis in serum-stimulated LNCaP prostate cancer cells in vitro. The present study tests the hypothesis that these effects on tumor cell lines are mediated by enhancement of the function of the p53 gene known to arrest cell growth and induce apoptosis. When LNCaP cells were cultured in exercise serum and compared with control serum, cell growth was reduced by 27%, and there was a similar 33% decrease in proliferating cell nuclear antigen protein, a marker for cell cycling. Apoptosis was increased by 371% with the exercise serum, and there was a 100% increase in p53 protein (75.2 +/- 2.0 vs. 38.2 +/- 2.0 pg/microg protein). When serum was used to stimulate LN-56 cells, a cell line with nonfunctional p53 derived from LNCaP, no significant reduction in cell growth or increase in apoptosis with the exercise serum was observed. These results indicate that exercise training alters serum factors in vivo that increase cellular p53 protein content and is associated with reduced growth and induced apoptosis in LNCaP prostate cancer cells in vitro.