Mouse macrophages and human peripheral blood lymphocytes exhibiting on the membrane autologous IgG as well as mouse thymocytes coated with human IgG, showed a strong cell concentration dependence of surface IgG disappearance during an incubation at 37 degrees C. Mouse spleen lymphocytes treated with anti-mouse Ig behaved in the same way regarding the disappearance of surface immunoglobulins, while the redistribution of ligands (cap formation) was not dependent on cell concentration.