The aim of this study was to examine the effect of caffeine on the activity of lysozyme and some immune parameters of mice. The mice were divided into five groups. Group 1, the control group, was given water. The other four groups were administered various concentrations of caffeine by oral intubation (group 2, 2 mg x kg(-1); group 3, 20 mg x kg(-1); group 4, 40 mg x kg(-1); group 5, 200 mg x kg(-1)). It was found that the activity of lysozyme in the serum depended on the caffeine dose. Compared with the control (group 1), lysozyme activity was 1.4-times higher in group 2, 1.6-times higher in the group 3, and 1.8-times higher in groups 4 and 5 (P < 0.05). In group 3 a significant increase in spleen weight was detected and the spleen index was 2.1-times (P < 0.05) higher compared with control. In group 3 the number of monocytes and neutrophils was 2.5-times higher (P < 0.05) compared with control. In group 5 the caffeine increased the number of neutrophils 2.7-times and increased the number of eosinophils 4.6-times (P < 0.05) compared with control. Our study revealed that caffeine played an important role in the development of protective immune response.