The in vitro effect of Na-salicylate on some functions of human mononuclear cells was studied. In therapeutical concentrations the drug was found to interfere both the function of lymphocytes and monocytes/macrophages. Na-salicylate in concentrations of 400-800 mug/ml slightly inhibited the digestion of yeast particles. When the drug was present in the culture medium in doses above 160 mug/ml during the cell differentiation period from 90 minutes to the 8th day of culture, a reduction in the number of adhesive, viable cells was recorded. The remaining cells, however, were found to have a normal phagocytic function. A strong and dose dependent inhibition of the ability of lymphocytes to proliferate after antigenic stimulation with BCG bacilli was recorded. The inhibitory effect on the PHA response, however, was less prominent. The results presented indicate that Na-salicylate has a direct inhibitory effect on lymphocyte proliferation and monocyte differentiation and phagocytosis, which may be part of the explanation of the anti-inflammatory effect of the drug.