Wild-type Chinese hamster V79 cells (6-thioguanine-sensitive) reduce the recovery of 6-thioguanine-resistant cells when they are cultured together at high densities, through a form of intercellular communication (metabolic cooperation). Cooperation is inhibited by 12-O-tetradecanoyl phorbol-13-acetate, rescuing the 6-thioguanine-resistant cells. These results may be useful in the study of an aspect of the mechanism of tumor promotion and in assaying for promoters.