Evidence was obtained that the mouse IgE antibody was absorbed in vitro by rat peritoneal mast cells, but not by macrophages. A significant absorption of the antibody was observed at 0°C in the presence of EDTA, although the rate of absorption was faster at 37°C than at 0°C. Mouse IgG1 and IgG2, which combine with mouse mast cells, did not combine with rat mast cells or macrophages at 0°C nor inhibit the binding of mouse IgE with mast cells.