Natural cytotoxicity of peripheral blood was assessed in adults with recurrent Herpes labialis and in patients with Herpes zoster. CD16+ cells count, lymphocytes forming conjugates with K 562 cells count and NK activity in chromium-release assay were measured. Decreased CD16+ cells count and depressed NK activity were found during latency of HSV infection. During reactivation of the infection lymphocytes forming conjugates count was increased. Functional activation of natural killing was noted in patients with herpes zoster in the acute phase of the disease. However, no differences between results of all NK tests after herpes zoster versus control group were found.