Wild-type Cloudman S91 melanoma cells have a retarded rate of division when agents which raise cyclic AMP levels such as melanotropin, protaglandin E1, or cholera toxin are supplemented to the culture medium. A mutant cell line was isolated which had the opposite response, i.e., the mutant grew very slowly unless agents which raised cyclic AMP levels were present (Pawelek et al., '75a). In this report evidence is presented indicating that the molecular basis for the mutant phenotype resides in the major cyclic AMP-dependent protein kinase found in the cells. The mutant kinase had increased thermolability and an elevated activation constant for cyclic AMP over the corresponding wild-type kinase. It is proposed that the elevated requirement for cyclic AMP for the proliferation of cAdep cells is related to the elevated activation constant of the kinase, suggesting that the kinase is a positive regulator of proliferation in Cloudman S91 cells.