A heterologous antihuman T-cell serum (anti-TH1), raised against purified peripheral T cells, and absorbed with an autologous Ig+ line, was shown to bind specifically to T- but not to B-lymphoid cells by both a complement-dependent cytotoxic assay and indirect immunofluorescence. Whereas 90% fetal thymocytes and thymocytes were killed by anti-TH1 and complement, a consistently restricted population (50-60%) of peripheral T cells from several normal donors were lysed, indicating that anti-TH1 is directed against one or more thymus-specific antigens which are lost or reduced on a subpopulation of human T cells in the periphery. Functional analysis of the unreactive (TH1-) and reactive (TH1+) T-cell subclasses demonstrated that TH1- cells mounted a good proliferative response to a battery of specific soluble antigens (mumps, PPD, tetanus toxoid) but neither responded in MLC, nor elaborated LMF in response to tetanus toxoid. In contrast TH1+ cells proliferated in MLC and elaborated LMF but did not respond by 3H-incorporation to soluble antigens. The relevance of these findings to human T-cell functions in vivo and to previously described functional subclasses of murine T cells is discussed.