Specific immunological responses (varicella-specific IgG and IgM) and various parameters (interferon, C3, C4, platelet count, erythrocyte count, erythrocyte CR1) in varicella infection were determined over the course of the disease (days) and their relationships to severity and immunological significance evaluated. Defining the day varicella appeared as disease day 0, IgG appeared on the disease day 4 and IgG on day 5. The preceding disease days were dominated by non-specific immunological mechanisms. Interferon appearance preceded that of these antibodies but did not correlate with severity. In serious cases, C3 and C4 increased in the acute stage while platelet count declined. Erythrocyte count decreased in severe cases after disease day 5. Three patients with lower erythrocyte CR1 activity values remained critical. These findings suggested that determining platelet count is expedient in determining severity in the early stage and that the complement pathway is a major component of early immunological response.