In the present study, the effect of $HgCl_2$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 $0.5{sim}5{mu}M;HgCl_2$and glass adherence, chemotactic activity and erythrocyte-antibody rosette forming activity were measured. $HgCl_2$ decreased the function of PMNs in all three aspects tested. $HgCl_2$significantly diminished glass adherence(40.5 {mu}M: 92{pm}12%$ (percentage of control, $mean{pm}$ S.D.); 41 {mu}M: 46{pm}11%,$ P<0.01; $3{mu}M: 35{pm}7%,$P<0.01;$5{mu}M:49{pm}10%,$ P<0.01). Similarly, significant differences were observed in chemotactic activity after $HgCl_2$treatment compared with control (control: $0.95{pm}0.14mm; 0.5 {mu}M: 0.91{pm}0.11 mm; 1 {mu}M: 0.77{pm}0.16mm, P<0.05; 3{mu}M: 0.61{pm} 0.06mm, P<0.01; 5{mu}M: 0.15{pm}0.03 mm, P<0.01).$ Also, 4HgCl_2$decreased the percentage of rosette-forming PMNs, indicating diminished phagocytic activity of PMNs upon $HgCl_2$ exposure compared with control (control: $58{pm}4%; 1{mu}M: 53{pm}4%, p<0.05; 3{mu}M: 49{pm}3%, P<0.01; 5{mu}M: 46{pm}3%, P<0.01).$ Cell viability was not antered after $HgCl_2$treatment (483{pm}5%$ viability in control PMNs versus $81{pm}8%$ viability in $5{mu}M$ Hg-treated PMNs), suggesting that the impaired PMN function after $HgCl_2$treatment was not due to nonspecific cytotoxicity induced by $HgCl_2$. $HgCl_2$-induced decrease in the function of PMNs may have some implications in depressed host susceptibilityupon bacterial challenge after mercury exposure.