Serum cholinesterase (CHE) activity was assessed in a group of patients with different types of viral hepatitis. Serum samples from 120 patients with such a diagnosis (37 with hepatitis A. 48 with hepatitis B and 35 with hepatitis C virus infection) were analysed. As compared to 66 serum samples from uninfected controls, the hepatitis C virus-infected patients showed significantly higher CHE activity (p < 0.00 1). Whereas diminished activity was detected in patients with either hepatitis A or hepatitis B virus infection (p < 0.001 for both groups). There was a correlation between CHE activity and other serum viral markers. The significance of this variation in CHE activity among different viral hepatitis infections should be established.