The natural antiviral immunity of human lymphocytes, leukocytes from peripheral blood and whole-blood cultures was studied using the method of infection with two viruses belonging to different taxonomic groups, vesicular stomatitis virus (VSV) and encephalomyocarditis virus (EMCV). The kinetics of virus replication in the kinds of cultures and the dependence of culture infection on pre-infection incubation time were studied. When the cultures were infected immediately after preparation, most of them were found to be resistant to the viruses. However, when they were infected after several (1-5) days of incubation, VSV and EMCV multiplied in the cultures to high titers. The time of losing resistance was individually differentiated. The results indicate the presence of a non-specific antiviral immunity characteristic for individuals. The antiviral immunity of healthy donors was compared with that of people suffering from recurrent infections of the upper respiratory tract. This latter group expressed statistically significant lower innate immunity than healthy donors. However, there were no differences in interferon (IFN) and tumor necrosis factor (TNF) production between these groups. In order to examine the contribution of the endogenous IFNs and TNF-alpha in maintaining innate immunity, specific antibodies against IFN-alpha, IFN-beta, IFN-gamma and TNF-alpha were added to VSV-infected leukocytes resistant to infection. The antibodies reduced the antiviral resistance in 9 of 16 experiments. The results suggest that both endogenous interferons and TNF-alpha may participate in the constitution of innate immunity, though they are not the only mediators of it.