In the present study, we examined the effects of over-expression of the potential tumor suppressor gene IGFBP-rP1/mac25 on cell-cycle kinetics in prostate cancer cells. The majority of the high expressing IGFBP-rP1/mac25 cell population was located in the G1 and sub-G0/G1 peaks; synchronizing cells in G2/M with nocodazole demonstrated the high expressing IGFBP-rP1/mac25 clones were delayed in the G1 phase of the cell cycle. Unscheduled expression of cyclin A in the sub-G0/G1 peak occurred in the IGFBP-rP1/mac25 clones. Immunoblots showed decreased cyclin D1 and p21 and increased cyclin E, p16, and p27 in the high expressing IGFBP-rP1/mac25 clones compared to the control cells. Cyclin D1/cdk-4,6 and cyclin E/cdk-2 kinase activities decreased but cyclin A/cdk-2 kinase activity increased for the high expressing IGFBP-rP1/mac25 clones compared to control cells. A pRb immunoprecipitation demonstrated more binding of E2F-1 to pRb in the high expressing IGFBP-rP1/mac25 clones than in control cells. Finally, cell senescence, as assessed by senescence-associated beta-galactosidase, demonstrated significantly more staining in the IGFBP-rP1/mac25 cells than control cells. These results suggest that IGFBP-rP1/mac25 alters the cell cycle kinetics of the M12 prostate cell line by delaying the cells in the G1 phase of the cell cycle. In addition, the appearance of cyclin A in the sub-G0/G1 phase of the cell cycle and the increased kinase activity of cyclin A/cdk-2 in the IGFBP-rP1/mac25 clones suggests that cyclin A is associated with the apoptotic cells.