Cell wall fraction of Listeria monocytogenes (LCWF), a B cell mitogen for mouse spleen cells, is also mitogenic for human adult and cord peripheral blood lymphocytes. Purified B-cell suspensions responded to LCWF in vitro proliferation, to a similar extent as the unfractionated suspensions. Furthermore, LCWF-induced B cell differentiation into IgM-containing cells and their percentage correlated significantly with the extent of lymphocyte proliferation.