Haemopoietic cells carry a variety of cell-surface molecules, some of which are known to have allotypic variation. In rats, the RT7 alloantigenic system has been well documented using alloantisera. We have produced the first mouse hybridomn cell line secreting an antibody. H1S41, which binds to leucocytes of rat strains carrying the RT7.2 but not the RT7.1 determinant. An lgG2b isotype switch variant (HIS4l.2b) of the original HIS41 (IgG1 isotype) was also made. HIS41 showed a clear and discrete binding in immunofluorescenl and histological experiments and has already been used in several studies on haemopoietic cell turnover and differentiation employing PVG rats congenic for RT7. The present study addresses the question of whether the RT7 gene products are members of the L-CA family, which has been a matter of controversy over the last decade. When using HIS41 for the analysis of tissue distribution and molecular weight of RT7 gene products, a strong similarity was evident with the data reported for the L-CA detected by MRC OX-1 and MRC OX-30. These two MoAb have been reported to bind to all members of the L-CA family. All haemopoietic cells, excluding erythrocytes and the more mature stages of erythropoiesis, stained with HIS41. The molecular weights of HIS41 binding molecules on thymocytes and peripheral T cells were comparable to the L-CA precipitated by MRC OX-1. Capping and sequential immunoprecipitation studies indicated that HIS41 and MRC OX-30- binding molecules were identical. MRC OX-1. however, appeared to bind only a subset of these molecules. Thus, our study confirms the identity of RT7.2 gene products and L-CA. lt also revealed a difference between MRC OX-1 and MRC OX-30 not noticed previously.