The alloantigenic specificity Ly-4.2 can be detected on a proportion of lymphocytes by the antiserum (BALB/c times SWR)F1 anti-B10.D2. In the preceding study it was shown that these lymphocytes were not thymus-derived (T) cells, as they were Thy-1 (theta)(-), and were therefore presumably B (bone marrow-derived) cells. Evidence is now presented for the reaction of the Ly-4.2 antiserum with functional B cells. Thus, the LY-4.2 and Thy-1.2 specificities were detected on antigen-binding rosette-forming cells (RFC) in mice both immune and non-immune to sheep red cells (SRC). RFC formed to endotoxin lipopolysaccharide (LPS) were also Ly-4.2(+). Memory cells to both SRC andLPS could be detected with anti-Ly-4.2 and anti-Thy-1.2 antisera, thereby indicating that both T and B cells are involved in memory to these antigens. Both direct and indirect antibody-forming cells (the PFC) could be inhibited, in vitro, by anti-Ly-4.2 antiserum, although it is likely that not all PFC are Ly-4.2(+). Neither of the specificities Ly-4.2 nor Thy-1.2 were detected on the bone marrow precursor of the splenic colony forming unit (the CFU). In an assay for B cells, the treatment of lymph node or spleen cells with anti-Ly-4.2 before transfer to irradiated recipients could inhibit the ability of these cells to make PFC to SRC, and this capacity could only be restored by bone marrow cells and not by thymus cells. These studies provide clear evidence for the presence of the Ly-4.2 specificity on antibody-forming cells and their precursor (B cells).