Several new aspects of the chemistry, genetics and cellular distribution of the murine Qa‐2 alloantigen were apparent in an analysis of this antigen using monoclonal antibodies recognizing a Qa‐2‐like antigen called Qa‐m2. Immunoprecipitation and sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis identified the Qa‐m2 alloantigen as a two‐chain structure composed of a 39 000 dalton heavy chain and a 12000 light chain which is probably β2‐microglobulin; the heavy chain was readily distinguished from that of H‐2. In addition, the expression of Qa‐m2 alloantigens on the cell surface was found to be controlled by the H‐2D gene with H‐2Db strains carrying approximately 8‐10 times greater amounts of Qa‐m2 than strains carrying the H‐2Dd or H‐2Dq alleles. Finally, the Qa‐m2 antigen, found predominantly on peripheral T cells, was present on only 10% of thymus cells. However, subpopulations of B cells (approximately 25% of all B cells) and bone marrow cells (15‐20%) were also reactive. The monoclonal anti‐Qa‐m2 antibodies differed in their reactions from that reported for the conventional anti‐Qa‐2 sera, which must, therefore, be complex. The monoclonal antibodies may be useful reagents for functional analysis of T and B cell subpopulations.