Recent studies have suggested that inhibition of the mitogen activated protein kinase (MAPK) pathway as well as abrogation of cell cycle check-point control can potentiate the lethal actions of chemotherapeutic drugs and radiation. We therefore investigated the impact of combined exposure to the check-point abrogator (UCN-01) in conjunction with MEK1/2 inhibitors upon survival of breast and prostate carcinoma cells. Treatment of cells with UCN-01 alone resulted in prolonged activation of the MAPK pathway. Inhibition of MEK1/2 caused modest reductions in basal MAPK activity and transiently suppressed UCN-01-stimulated MAPK activity below that of MEK1/2 inhibitor alone. Significantly, combined, but not individual, exposure of cells to UCN-01 and MEK1/2 inhibitors enhanced BAX association with mitochondria and triggered release of cytochrome c into the cytosol, accompanied by activation of effector pro-caspases, resulting in a greater than additive potentiation of apoptosis within 1 8-24h. Radiation exposure of drug treated cells did not further enhance apoptosis. Treatment of cells with both caspase 9 and caspase 8 inhibitors was required to completely inhibit apoptosis in carcinoma cells. Overexpression of Bcl-(xL) blocked cytochrome c release and cell killing induced by the drug combination. Colony forming assays demonstrated that cells exposed to both agents exhibited a substantial reduction in clonogenic survival compared to either drug alone; moreover, radiation further reduced clonogenic survival despite failing to promote additional apoptosis. Collectively, these data demonstrate that combined exposure of carcinoma cells to UCN-01 and MEK1/2 inhibitors induces apoptosis and interacts with radiation to further reduce clonogenic survival.