A new antihuman T cell monoclonal antibody, MAb 22, recognizes an antigen that is present on all mature T cells, but detected on only a subset of thymocytes. Dual color flow microfluorimetry (FMF) demonstrates that MAb 22 staining has concordant distribution with pan-T MAb specific for CD3 and CD5 and includes the CD4 and CD8 subsets; other FMF studies confirm T cell specificity of MAb 22 expression by cells of hematopoietic origin. The antigen recognized by MAb 22 is expressed on only a subset of thymocytes and by dual FMF is expressed at a mature thymocyte stage. Immunoprecipitation by MAb 22 demonstrates a series of molecules with a predominant 45 KD protein and associated 12 and 95 KD proteins under reducing conditions, while a 92 KD protein predominates under nonreducing conditions. Comparisons of expression on a variety of T cell lines, including a mutant line defective in the expression of the T cell receptor, by FMF analysis further distinguishes the determinant recognized by MAb 22 from those detected by MAb of defined T cell specificity.