In this work we have investigated the changes in T-helper and T-suppressor cells and T-cell proliferative response to phytohemagglutinin (PHA) in mercury-exposed workers. The study group consisted of 33 workers from a mercury-producing plant with a mean age of 29 years and a mean exposure period of 19 months. At the time of testing, and for the three previous months, the exposed population had urinary mercury levels below the currently accepted limit of 50 micrograms/g creatinine. A reverse CD4+/CD8+ ratio was observed in the mercury-exposed individuals which was characterized by a reduction in the number of CD4+ lymphocytes. No changes were observed in the proliferative response of lymphocytes from exposed individuals to PHA. Similarly, no proliferative response was observed when lymphocytes from normal individuals were cultivated in the presence of serum from the exposed workers. We found no correlations between lymphocytes changes and urinary mercury concentrations, time of exposure or the age of the workers.19499-510