A vitamin A analog, 13-cis-retinoic acid (13-cis-RA), was tested for effect on intercellular metabolic cooperation between 6-thioguanine-resistant and -sensitive V79 Chinese hamster cells. Most typical tumor promoters have been reported to inhibit metabolic cooperation. 13-cis-RA was found to enhance metabolic cooperation of cells treated with a potent tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate. However, inhibition of metabolic cooperation was noticed when 13-cis-RA was used at higher concentrations.