The ability of cell suspensions from spleen to bind antibody raised against β-galactosidase of Escherichia coli was tested in vitro. To exclude macrophage binding most macrophages were removed by their adherence property from the spleen cell suspensions, and the lymphocyte-rich fraction was used to test for cytophilic antibody. Hundreds of 0.2-µl droplets containing 30 to 40 cells were made in oil chambers; the number of such cells was chosen so that macrophages would be statistically excluded from one-third of the total drops. The antigen-binding activity of cells treated with antiserum in those droplets containing no macrophages was calculated, and lymphocytes from spleen were shown to bind cytophilic antibody. In addition, thymocytes were also able to bind antibody and the binding as shown to be inhibited by exposing the cells to normal mouse serum. This indicated that the receptors for cytophilic binding on thymus cells could be preempted by molecules present in normal serum. These observations are consistent with the assumption that cytophilic antibody plays a role in lymphocyte-antigen interactions and may thereby influence the regulation of the immune response.