A431 cells, a human epidermoid carcinoma cell line, express an unusually large number of cell surface receptors for the epidermal growth factor (EGF). The growth rate of A431 cells was estimated by measuring [3H]thymidine incorporation at the logarithmic growth phase. The growth of the cells in protein-free medium was partially inhibited by exposure of the cells to pertussis toxin, islet-activating protein (IAP). The growth in both serum-containing and protein-free medium was inhibited by high concentrations of EGF, and these inhibitions were partially reversed by treatment of the cells with IAP. The effects of IAP were well correlated with the degree of ADP-ribosylation of a membrane 40-kDa protein. Thus, IAP sensitive G-proteins appear to be involved in the signal transduction of both positive and negative regulation of A431 cell growth. The possibility is also discussed that phosphatidylinositol turnover may participate in growth regulation.