Purified peripheral blood human T lymphocytes, derived from normal individuals, were assayed for their susceptibility to low doses of ultraviolet B (UVB) in vitro. Exposure of T cells to graded single doses (range 0-8 mJ/cm2) of UVB resulted in a dose-dependent reduction of viability. This phototoxic effect was not immediately apparent, however, but became manifest 48-72 h subsequent to irradiation. A dose as little as 0.5-1 mJ/cm2 was sufficient to cause 50% mortality. Irradiated T cells showed a reduced ability to proliferate, irrespective of the stimulus used, and a reduced ability to produce cytokines IL-2, IL-4, IL-5, interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha). This decreased ability was UVB-dose related and, remarkably, was exactly correlated to phototoxicity. UVB had no effect on CD4 and CD8 expression or their ratio, whereas the expression of IL-2R (CD25) was only slightly reduced. Our data suggest that UVB radiation neither selectively affects Th1 or Th2 nor CD4 or CD8 T cell subsets. The high susceptibility of T cells to UVB might explain, at least in part, the beneficial effect of phototherapy during treatment of certain immunodermatological diseases.