The expression of HLA-DC antigens on peripheral blood mononuclear cells, in tonsil and lymph node tissue sections, on tumor cell lines, and on activated T cells was studied using monoclonal antibody, anti-Leu 10. Anti-Leu 10 reacts with HLA-DC molecules on homozygous B cell lines expressing HLA-DR 1,2,4,5,6,8, and 9. It reacts with heterozygous B lymphocytes expressing DR7 and DRw10, suggesting it also recognizes HLA-DC molecules linked to DRw10. The HLA-DC molecules detected by anti-Leu 10 are expressed on all Ig-positive and DR-positive peripheral B lymphocytes and an apparent subpopulation of DR-positive peripheral blood monocytes. Two-color immunofluorescence experiments using phycoerythrin-anti-HLA-DR (L243) and FITC-anti-Leu 10 demonstrated a correlation of the amounts of HLA-DR and DC antigens expressed on B lymphocytes. Cells expressing relatively low, or high amounts of one Class II molecule express respectively low or high amounts of the other Class II molecule. Anti-Leu 10 reacted with all B lymphocyte derived tumor cell lines not with lines of myeloid or erythroid origin, and with only one T cell derived line, HUT-78 which has an activated T cell phenotype. Consistent with this result, anti-Leu 10 binding suggested the presence of HLA-DC on activated T cells in lymphoid tissue, in addition to staining B cells. HLA-DC was also detected on mitogen and MLC activated T cells by anti-Leu 10 binding. Anti-Leu 10 is, therefore, a useful reagent for further studies of the role of HLA-DC in T cell activation and in normal B cell and monocyte functions.