Protein synthesis is normally required for G2-cell progression and for recovery from radiation-induced G2-arrest. In the presence of 5 mM caffeine this requirement is alleviated, indicating that the mechanism responsible for G2 cell progression actually remains intact in irradiated or protein synthesis inhibitor-treated cells. It is suggested that both radiation and cycloheximide-induced G2-arrest are not, therefore, passive consequences of cellular defects, but are rather, active cellular responses to the state of cellular integrity, implying the existence of G2 cell progression controls.