1 alpha-dihydroxivitamin D3 [calcitriol; 1,25-(OH)2D3], the most biologically active metabolite of vitamin D3, exerts several effects on peripheral blood mononuclear cells (PBMC). We report here the effects of calcitriol on PBMC proliferation and on the expression of some lymphocyte surface differentiation markers, as well as its action on lymphokine production. Calcitriol inhibited the proliferation of PHA-activated PBMC in a dose-dependent manner, with peak activity at 10(-8) M. Exposure of PHA-stimulated PBMC to 10(-8) M calcitriol for 3 days tended to increase the percent of CD4- and CD8-positive cells, though statistical significance was not reached. A more striking effect of calcitriol was seen on the expression of the non-polymorphic determinants of HLA class II DR molecules; in cultures stimulated with PHA for 3 or 4 days; 10(-8) calcitriol doubled the percent of DR-positive cells as compared to controls treated with PHA alone. This activity peaked at 10(-9) M, a supra-physiologic dose. After 3 days in culture, 10(-8) M calcitriol strongly inhibited the production of both IL-2 and IFN-gamma. This effect was evident at different PHA concentrations (0.5, 1.5 and 3.0 micrograms/ml), and almost disappeared at 10(-10) M. These results underline the immunoregulatory role of calcitriol, but well defined experimental models in vitro are needed for elucidating the relevance of this compound in physiology and, possibly, in therapeutics.