High molecular weight lymphocyte surface proteins are structurally related and are expressed on different cell populations at different times during lymphocyte maturation and differentiation.

HMW lymphocyte surface proteins T20 and B220, which are expressed on T and B cells, respectively, have been found to be extensively related in structure, beyond what would be expected by the sharing of the determinant detected by monoclonal antibody 13/2. In addition, another HMW protein (T200A) has been discovered, which is preferentially expression proliferating T cells, and which exhibits significant structural homology with T200, but does not express the 13/2 determinant. T200A bears a novel antigenic determinant, detected by monoclonal antibody BS-S28E, which is not shared by either T200 or B220. A fourth HMW protein (170/100) has been found, which is expressed on most lymphocytes, but which is neither structurally nor antigenically related to T200A, T200, or B220. Monoclonal antibodies reactive with 170/100 inhibit T cell-mediated cytolysis, although monoclonal antibodies reactive with T200A, T200, or B220 do not.