A large number of studies have been undertaken in an attempt to define the mechanism by which caffeine (1,3,7-trimethylxanthine) reduces the duration of radiation-induced arrest of cells in the G2 phase of the cell cycle. These studies are summarized and those agents which mimic the action of caffeine are listed in the order of their potency. This ranking does not match any activities of these agents described previously, but provides a comparison for future studies, which might profitably include measurement of the ability of these agents to inhibit protein kinases.