In the present report, we characterize a monoclonal antibody directed at a surface differentiation antigen on human T cells. The monoclonal antibody, OKT17, recognizes a cell surface antigen present on the majority of resting normal peripheral T cells. In contrast, OKT17 is unreactive with normal B cells, B cell lines, T cell lines, or SIg+ CLL. Interestingly, after activation, the antigen recognized by OKT17 is lost from a subset of OKT4+ cells. We took advantage of this finding to explore further the functional heterogeneity within activated OKT4+ cells. Evidence was obtained that the PWM-activated OKT4+ subset remaining after depletion of OKT17-reactive T cells (OKT4+ 17-) contains radiosensitive helperr cells but is devoid of suppressor cells. In contrast, the activated OKT4+ 17+ population contains potent radiosensitive suppressor cells as well as radioresistant helpe cells. Taken together, these studies suggest that the OKT17 monoclonal antibody can differentiate two functionally mature, activated OKT4+ human T cells: OKT4+ OKT17+ radiosensitive suppressor cells and OKT4+ 17- radiosensitive helper cells.