The monoclonal antibody (MoAb) 6/31, an IgG2a, and the MoAb Cr-1, an IgGl, identify determinants expressed on the heavy chain of subpopulations of HLA-A and B antigens while the MoAb NAMB-1, an IgGl, reacts with human β2-microglobulin (β2-m). The MoAb 6/31 and CR-1 reacted with lymphocytes from all of the 20 HLA-typed donors and the 6 homozygous lymphoid cell lines tested, indicating that the corresponding determinants are different from those defining the conventional serological polymorphism. The distribution of the determinants recognized by the MoAb 6/31 and CR-1 on molecules carrying HLA-A and B allospecificites was investigated by incubating NP-40 extracts of cultured lymphoid cells with insolubilized monoclonal antibodies and then measuring the level of HLA-A and B antigens in the supernatant: these studies showed that the determinant defined by the MoAb CR-1 is expressed on all HLA-A and B allospecificities tested, while that defined by MoAb 6/31 is detectable only on some of the HLA-A and B allospecificities. Inhibition-binding assays with radiolabeled monoclonal antibodies and Fab2-blocking assays indicate that the antigenic determinant identified by the MoAb CR-1 is spatially close to those recognized by the MoAb 6/31 and by the MoAb NAMB-1; the latter two antibodies react with spatially distant antigenic determinants. Furthermore, Fab2-blocking assays indicate that the determinants recognized by each monoclonal antibody is spatially close to the conventional serologically defined allotypic determinants only on some of the HLA-A and B allospecificities tested, and that sera with the same specificity may recognize different determinants on a given HLA-A and B alloantigen.