BHK cells blocked at any of several points in the cell cycle override their drug-induced arrest and proceed in the cycle when exposed concurrently to the protein kinase inhibitor 2-aminopurine (2-AP). For cells arrested at various points in interphase, 2-AP-induced cell cycle progression is made evident by arrival of the drug-treated cell population in mitosis. Cells that have escaped from mimosine G1 arrest, from hydroxyurea or aphidicolin S-phase arrest, or from VM-26-induced G2 arrest subsequently have all the hallmarks of mitosis--such as a mitotic microtubule array, nuclear envelope breakdown, and chromatin condensation. In a synchronous population, the time course of arrival in mitosis and its duration in 2-AP-treated cells that have escaped drug-induced cell cycle blocks is indistinguishable from control cells. Cells arrested in mitosis by nocodazole or taxol quickly escape mitotic arrest and enter interphase when exposed to 2-AP. 2-AP by itself does not influence the timing of cell cycle progression. We conclude that 2-AP acts to override checkpoints in every phase of the cell cycle, perhaps by inhibiting a protein kinase responsible for control of multiple cell cycle checkpoints.