1,25-dihydroxyvitamin D3(1,25–(OH)2D3), the biologically active form of vitamin D3, has been shown to modulate lymphocyte functionsin vitro. These effects are exerted through binding to specific receptors that are expressed in activated, but not in resting lymphocytes. 1.25–(OH)2D3inhibits lymphocyte proliferation, immunoglobulin production and the release of cytokines including interleukin-2 (IL-2) and interferonγ(IFNγ) by mitogen driven blood mononuclear cells (MNC). A distinction between CD45RA+ and CD45R0+ subsets of T cells has, however, proven extremely relevant in terms of immunoactivation and immunopathology. The present study was undertaken to evaluate effects of 1,25–(OH)2D3on proliferation and cytokine production by purified CD45RA+ and CD45R0+ T cells.1,25–(OH)2D3caused a dose- and time-dependent reduction in phytohemaglutinin-(PHA) and poke-weed mitogen (PWM)-driven proliferation of purified CD45R0+ T cells. In contrast, proliferation of the CD45RA+ subset was unaffected by this treatment. Comparable levels of lymphotoxin (LT), IFNγand IL-2 were obtained in cultures of both subsets. 1,25–(OH)2D3reduced these levels, but the suppressive effect of the hormone was delayed in cultures of CD45RA+ T cells. The results suggest that the CD45RO+ subset is relatively more sensitive than CD45RA+ subset to the inhibitory effects of 1,25–(OH)2D3. This finding may be of pharmacological interest, because the CD45R0+ subset plays a key role in immune activation and because these cells have been associated with the pathogenesis of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.