HLA-DR antigen is constitutively expressed on antigen-presenting cells, such as B lymphocytes, monocytes and dendritic cells, but it can also be detected on activated T lymphocytes and activated granulocytes. Antibodies to HLA-DR antigen are together with a panel of other antibodies considered essential for the initial evaluation of acute leukemia, chronic T and B-cell leukemia, and myeloid leukemia. The reagent is not intended for use in tissue typing.