The introduction of a bacterial aminoglycoside phosphotransferase gene (neo gene) into A-431 cells was found to result in disappearance of high-affinity binding sites of the epidermal growth factor receptor (EGFR), probably by affecting the phosphorylation level of the receptors. Using A-431 cells and their neo gene-transfectants, we studied the relation between "rounding" and the high-affinity sites for EGF; and we also examined the role of protein kinase C (PKC) and A (PKA) in the EGF-induced cell rounding. Pretreatment of A-431 and their transfectant cells with 12-O-tetradecanoylphorbol 13-acetate (TPA; 100 ng/ml), an activator of PKC, for 30 min inhibited both the EGF-induced cell rounding and expression of high-affinity binding sites for EGF. However, both of these responses were recovered when cells were pretreated with TPA for 20 h, which treatment is known to result in depletion of PKC by a process called "down regulation". A similar recovery was also observed when cells were pretreated with forskolin (100 microM), an activator of PKA, for 30 min. Both cell rounding and EGFR high-affinity binding sites disappeared by activation of PKC, and reappeared by activation of PKA. These results suggest that the rounding of A-431 cells by EGF was induced via the high-affinity binding sites of EGFR.