Bone marrow of both normal and rearrangement-deficient mice contains a small population of B220(CD45R)+ cells, which do not express the B lineage marker CD19. Instead, part of this population coexpresses the surface marker CD43 and lacks or expresses very low levels of heat stable antigen (HSA) and BP-1, thus representing a part of Hardy's fraction A (B220(+)-CD43+HSA-, BP-1-) of B lineage development. However, some 20-40% of these B220(+)-CD19- cells also coexpress the NK1.1 surface molecule and do not express genes like VpreB or B29 restricted to the B cell lineage. These cells respond to recombinant interleukin 2 in vitro, and develop into killer cells that can lyse the prototypic NK target tumor cell, YAC-1, as well as syngeneic normal lipopolysaccharide or concanavalin A blasts, providing they lack the surface expression of major histocompatibility complex class I molecules. The implications of these findings for studies on B lymphopoiesis are discussed. It is suggested that the CD19-specific monoclonal antibody is more reliable, as in humans, than B220(CD45R) to detect B lineage cells in mice.