In hematopoietic cells, gamma-irradiation causes a p53-dependent transient G1 phase cell cycle arrest. Various extracellular growth inhibitory signals elicit G1 arrest by targeting CDK4. Here we show that in a myeloid cell line, 32D cl 3, enforced expression of CDK4, but not cyclins D2 nor D3, overrides the gamma-irradiation-induced G1 arrest. CDK4 does not confer resistance to the radiation-induced G2 block observed in parental cells. Ectopic expression of CDK4 overcomes the ionizing radiation-induced inhibition of CDK4 and CDK2 kinase activity. The levels of CDK4 protein do not change after exposure to ionizing radiation in either parental cells or those overexpressing CDK4. Ionizing radiation induces the expression of both p53 and p21, and in cells constitutively synthesizing exogenous CDK4, the return of p53 protein levels to baseline is prolonged. Increased levels of p21 are found associated with CDK4, and not CDK2, in the lines overexpressing CDK4, compared to the parental line, after exposure to ionizing radiation. Enforced expression of CDK4 may therefore overcome a gamma-irradiation-induced G1 arrest through the titration of the CDK inhibitor p21 allowing both CDK4 and CDK2 to remain active.