In this report, we explored the functional heterogeneity within the OKT4+ subset of human T cells. Evidence was obtained that although in vitro pokeweed mitogen-activated OKT4+ cells can function as radioresistant helper cells, these activated OKT4+ cells could also exert potent feedback suppression. Despite the induction of suppressor cells after pokeweed mitogen activation, the OKT4+ population maintains its original OKT3+, OKT4+, nd OKT8- surface phenotype. The suppressor cells contained within the activated OKT4+ population were found to be radiosensitive. Importantly, the suppression mediated by activated OKT4+ cells required the presence of radiosensitive cells contained within the resting OKT4+ population. Taken together, these results suggest that the OKT4+ subset of human T cells contains cells that can be activated to differentiate into suppressor cells independent of OKT8+ cells.