The effect of zinc chloride (ZnCl₂) upon the proliferation of human peripheral lymphocytes in vitro was investigated. Zinc chloride in four concentrations (1 μM, 10 μM, 50 μM, 100 μM) was added to the lymphocytes cultures. Results indicate that high concentrations of ZnCl2 (100 μM) slightly inhibit the proliferation of human peripheral lymphocytes in vitro. The phase of lymphocyte proliferation cycle affected by ZnCl₂ was also investigated. For this purpose ZnCl₂ in two concentrations (50 μM, 100 μM) was added to the cell culture in 4 different time points: at the start of the cell culture, after 4 hours, 24 hours and 48 hours of incubation. The sooner ZnCl₂ was added the stronger inhibition of lymphocytes proliferation was observed; the greatest change was given by 100 μM concentration of ZnCl₂.