Forty-nine patients with acute viral hepatitis A and B were examined for the relationship of the blood content of individual lymphocyte subpopulations at the height of the disease to subsequent duration of the cytolytic syndrome (high activity of ALT). The duration of the cytolytic syndrome associated with viral hepatitis A was reversely related to the content of B lymphocytes and when associated with viral hepatitis B to the content of T cells--immunoregulator precursors (autorosette-forming cells). In both patterns of viral hepatitis, the duration of the cytolytic syndrome did not depend on the rate of lymphocyte sensitization to the inducer antigens (HAVAg, kHBsAg). Unlike viral hepatitis A, the duration of the cytolytic syndrome in viral hepatitis B was directly related to the rate of lymphocyte sensitization to hepatic lipoprotein.