In the present study, the effect of HgCl₂ on the function of human peripheral polymorphonuclear leukocytes(PMNs) was examined. PMNs were isolated from human peripheral blood with density centrifugation in Ficoll-Paque. The cells were then incubated with 0.5∼5 μM HgCl₂ and glass adherence, chemotactic activity and erythrocyte-antibody rosette forming activity were measured. HgCl₂ decreased the function of PMNs in all three aspects tested. HgCl₂ significantly diminished glass adherence(0.5 μM: 92±12% (percentage of control, mean±S.D.); 1μM: 46±11%, P<0.01; 3 μM: 35±7%, P<0.01; 5 μM: 49±10%, P<0.01). Similarly, significant differences were observed in chemotactic activity after HgCl₂ treatment compared with control (control: 0.95±0.14 mm; 0.5μM: 0.91±0.11 mm; 1μM: 0.77±0.16 mm, P<0.05; 3μM: 0.61±0.06 mm, P<0.01; 5μM: 0.15±0.03 mm, P<0.01). Also, HgCl₂ decreased the percentage of rosette-forming PMNs, indicating diminished phagocytic activity of PMNs upon HgCl₂ exposure compared with control (control: 58±4%; 1 μM: 53±4%, P<0.05; 3 μM: 49±3%, P<0.01; 5 μM: 46±3%, P<0.01). Cell viability was not altered after HgCl₂ treatment (83±5% viability in control PMNs versus 81±8% viability in 5 μM HgCl₂-treated PMNs), suggesting that the impaired PMN function after HgCl₂ treatment was not due to nonspecific cytotoxicity induced by HgCl₂. HgCl₂-induced decrease in the function of PMNs may have some implications in depressed host susceptibilityupon bacterial challenge after mercury exposure.