The authors present a case of chronic hepatitis presumably due to Syncumar treatment. The patient should be maintained on anticoagulant therapy due to combined prosthetic valve insertion. Chromatin activation of blood lymphocytes was investigated to justify the drug induced liver injury. In this test system Syncumar and other anticoagulant derivates gave a positive reaction to a different extent. The increased serum bilirubin and transaminase levels, which indicated liver injury showed regression in the course of the treatment and after changing Syncumar to Marcoumar (Phenprocoumon).