An heterologous antiserum specific for bone marrow-derived cells (B cells) was prepared by immunizing rabbits with lymph node cells from nude mice. After absorption with mouse red blood and thymus cells, the antiserum killed a population of cells from various lymphoid organs and the cytotoxic effects were inversely related to those of anti-theta antibody. When bone marrow or spleen cells were treated with the antiserum and guinea pig complement before transfer into irradiated mice, the number of plaque-forming cells was greatly reduced in the spleen of the recipient. Pretreatment of thymus cells with the antiserum in a similar way resulted in no inhibition of hemolytic plaque. When spleen cells from mice previously immunized with sheep red blood cells were treated with the antiserum and complement, the formation of hemolytic plaque was not affected. These findings indicated that the antiserum was specific for B cells and that the number of antigenic determinants on B cells to which the antiserum reacted decreases during differentiation into antibody-forming cells.