We studied DNA damages (single-strand breaks and alkali-labile sites) in peripheral blood lymphocytes from patients with chronic viral hepatitis and cirrhosis of mixed etiology. The structure of DNA was estimated fluorometrically by changes in the intensity of ethidium bromide fluorescence. Monoinfection with hepatitis B and C viruses was not accompanied by considerable changes in DNA structure in peripheral blood lymphocytes from patients with chronic diseases. The incidence of DNA damages in lymphocytes increased in patients with hepatitis G virus and TTV monoinfection. This is probably related to replication of these viruses in nucleated blood cells. Our results suggest that hepatitis C virus potentiates damaging effect of hepatitis G virus on DNA in lymphocytes.