Lymphocyte differentiation, maturation and peripheral functions are affected by the thymic protein hormone thymulin. Mercury at very low concentrations has been seen to impair some lymphocytic functions causing subclinical manifestations in exposed workers. The present study was performed to test in vitro the effect of mercury on the production kinetics of thymulin using cultures of whole thymuses from young mice. Exposure to mercury (10(-8) M and 10(-6) M) added to the cultures, reduced kinetic thymulin production at all time intervals considered (1, 2, 4, 5 and 6 h) as compared to kinetic thymulin production of thymuses from young control mice. After the first hour the inhibition is more evident at the highest mercury concentration. Thymulin production decreased by 70, 74, 82 and 86% and by 55, 66, 73 and 81% for mercury concentrations of 10(-6) M and 10(-8) M, respectively, after 2, 4, 5 and 6 h. Mercury toxic effect on thymulin kinetics may be directly exerted to thymulin synthesis in epithelial cells, although it is less dramatic than that of cycloheximide (CHX), known as a potent inhibitor of protein synthesis in such cells. The toxic effect of mercury on thymic endocrine activity might cause the subclinical effects on cell-mediated immunological status observed in mercury exposed workers.