AbstractInternalization of epidermal growth factor (EGF) receptors by human A431 epidermoid carcinoma cells was studied at various culture densities. The extent of EGF receptor internalization was measured by quantitation of internalized125I-EGF during incubation at 37°C for 30 min. When cell culture density was below 1×105cells/cm2receptor internalization was active; 30-40% excess moles of ligand over the moles of surface EGF receptors were internalized during this period. However, when culture density increased to above 1.5×105cells/cm receptor internalization became less extensive, as only 30-50% of ligand bound to the cell surface underwent internalization during 30 min incubation. In parallel with this reduction in receptor internalization, the degradation rate of35S-methionine labeled EGF receptors was reduced at a high culture density. In contrast with this regulation of receptor internalization, the affinity of EGF receptors for the ligand increased as culture density increased. The extent of EGF-dependent receptor phosphorylation was found to be constant at all culture densities tested. Thus, the observed low level of receptor internalization at high culture densities was not attributable to lower responsiveness of receptors to EGF. These data suggest the presence of an as yet unidentified cell density-dependent mechanism for regulating receptor internalization in cultured A431 cells.