C.B-20 (Ighb) but not BALB/c (Ighb) mice are able to reject BCL1, a spontaneous B cell leukemia of BALB/c origin. The protective immune response is directed to ward the minor histocompatibility (H) antigen, H-40, which can be detected by cytotoxic T lymphocytes and is expressed on surface (s) immunoglobulin positive tumor cells including BCL1as well as sIg+lymphoblasts activated by lipopolysaccharide. The fact that only sIg+tumor cells and lymphoblasts express H-40 suggests that sIg itself may be a component involved in CTL recognition of this antigen. However, this possibility was ruled out by demonstrating that removal of sIg from target cells did not prevent H-40-specific CTL recognition. To determine whether H-40 expression was coordinately regulated with that of membrane Ig, we determined whether H-40−sIg−cells acquire H-40 when induced to express sIg. We also transfected a functionalcandvregion gene into a sIg−lymphoma so that it acquired a slg+phenotype. In neither case did such slg+cells acquire H-40. Together, these data indicate that H-40 is expressed on B cells representing a particular stage of differentiation that is coincidental with slg.We previously showed thatH-40andIghloci are linked on the 12th murine chromosome. In this study we further localizedH-40to a point beyondAat(formerlyPre-1) nearLm-1, a locus previously described to encode minor H-antigens expressed on lymphomyeloid cells. Thus,H-401and Lm-1 may represent a gene cluster encoding antigens expressed on subsets of lymphoid and myeloid cells.