Serological and immunochemical assays have shown that the monoclonal antibody Q1/28 recognizes an antigenic determinant which is expressed on the heavy chain of subsets of HLA-A, B antigens and is distinct from those defining the serological polymorphism of this system. Association of the HLA-A, B heavy chain with beta 2-microglobulin is not required for expression of the antigenic determinant recognized by the monoclonal antibody Q1/28, since this antibody can immunoprecipitate a 45 000 m.w. component from radiolabeled lymphoid-cell glycoproteins immunodepleted with either an anti-human beta 2-microglobulin xenoantiserum or the MoAb W6/32 to framework determinants of HLA-A, B, C antigens. Furthermore, the MoAb Q1/28 can immunoprecipitate a 45 000 m.w. component from an NP40 lysate of radiolabeled Daudi cells, which lack the genetic information for beta 2-microglobulin. The determinant recognized by the MoAb Q1/28 is relatively resistant to denaturing treatments and does not appear to be carbohydrate in nature. The MoAb Q1/28 is the first example of an antibody which recognizes an antigenic determinant expressed on both the beta 2-microglobulin-associated and free HLA-A, B heavy chains.