We have investigated the number of B lymphocytes in mercury-exposed workers. The study group consisted of 33 workers from a mercury-producing plant, mean age 27 years and a mean exposure period 19 months. At the time of testing and for the three previous months, the exposed persons had urinary mercury levels below the currently accepted limit of 50 micrograms g creatinine. A significant reduction in the number of B lymphocytes was observed in the mercury-exposed individuals. We found no correlation between B lymphocytes changes and urinary mercury concentrations, length of exposure or age of the workers.81130-3