The chemotactic and nitroblue tetrazolium reducing activities of neutrophils from 48 mercury-exposed workers were examined and compared with those of non-exposed, age- and sex-matched individuals. At the time of testing, the exposed population had a mean (+/- s.d.) urinary mercury concentration of 24.0 +/- 20.1 micrograms g-1 creatinine and in 44 of these workers urinary mercury levels were below the accepted threshold level (TLV) of 50 micrograms g-1 creatinine. The two neutrophil functions were significantly reduced in the mercury-exposed workers compared with the controls. In 28 of these workers, chemotaxis was re-evaluated 6 months later. During the intervening 6 months, the level of hygiene was improved throughout the plant and urinary mercury concentrations were determined monthly in each worker. Despite a significant reduction in urinary mercury concentrations, neutrophil migration did not return to within the normal range. These results suggest that 'safe' level mercury exposure may lead to impairment of neutrophil function.14281-6