Antipain (AP) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) were tested in V79 Chinese hamster cells for cytotoxicity and effects on survival and 6-thioguanine-resistant (6-TGr) mutation after UV-irradiation. AP and/or TPA were relatively non-cytotoxic and had no significant effects on UV survival. Despite their non-mutagenicity, the recovery of UV-induced 6TGr colonies was significantly enhanced by the pretreatment with either AP (0.5-2 mM) or TPA (0.1-1 microgram/ml) only during the expression period before the 6TG selection at a low density of cells in the absence of AP or TPA. Such enhancing effects were maximal when AP or TPA was present during the late expression period after the mutation fixation and extensive dilution of DNA lesions. Reconstruction experiments revealed the antagonistic actions that TPA and AP tended to eliminate and increase, respectively, the metabolic co-operation. In the TPA-plus-AP treatment, AP abolished the TPA-enhanced recovery of induced mutants. Thus, it seems that TPA increases the mutant recovery largely through decreased metabolic co-operation and AP could modulate the mutation expression. Further, an error-prone inducible repair may not exist, or, if it exists, AP may not inhibit it in V79 Chinese hamster cells.