A quantitative analysis by flow cytometry of the rate and extent of endocytosis of ligands bound to the membrane immunoglobulins of mouse B-splenocytes is reported. The temperature dependence and the response to inhibitors of oxidative metabolism are described. Inhibitors of the cytoskeleton (cytochalasin B, vinblastine and colchicine) and of calmodulin (trifluoperazine) do not interfere with endocytosis at non-lethal doses. Similarly fetal calf serum does not modify the rate and extent of the process. Endocytosis occurs in a similar time range, but to a lesser extent, when the ligand is monovalent than when it cross-links the membrane immunoglobulins. Finally, it is shown that, within minutes after its internalization, the divalent ligand is found in an acidic environment, while the monovalent ligand is not. These results, in agreement with the model of receptor recycling, suggest that the divalent ligand-receptor complex is indeed internalized and captured in an acidic environment while the monovalent ligand-receptor complex is internalized and probably rapidly recycled back to the cell surface.