Interactions between membrane proteins are believed to be important for the induction of transmembrane signaling. Endocytosis is one of the responses which is regulated by both intracellular and extracellular signals. To study such interactions, we have measured the lateral mobility and rate of endocytosis of epidermal growth factor receptor in three transfected NIH-3T3 cell lines (HER84, HER22, and HER82) expressing 2 X 10(4), 2 X 10(5) and 1.5 X 10(6) EGF-receptors per cell, respectively. Using rhodamine-labeled EGF (Rh-EGF) and rhodamine-labeled monoclonal anti-EGF-receptor antibody (Rh-mAb-108), we measured twofold decreases in the lateral diffusion coefficients for each approximately 10-fold increase in EGF-receptor concentration. Since steric effects cannot account for such dependence, we propose that protein mobility within the membrane, which is determined by the rate of motion between immobile barriers, decreases due to aggregate formation. The rate of endocytosis also decreases twofold between the HER84 (2 X 10(4) receptors/cell) and HER22 (2 X 10(5) receptors/cell) cell lines, suggesting that it is diffusion limited. The comparable rates of endocytosis of the HER82 and HER22 cell lines suggest that at high receptor density endocytosis may be limited by the total number of sites for receptors in coated-pits and by their rate of recycling.