The positivity of the anti-HCV antibody has been studied by means of an immuno-enzymatic solid phase method, on 1.605 blood samples. They were drawn from 5 groups of people, during the period from February 1 to October 31, 1992: a) all blood donors who made the donation at the Transfusion Service of Tivoli Hospital during that period; b) all intravenous drug users who came to Tivoli Hospital for control; c) all patients in the Contagious Disease Section with suspected liver disease, always during the same period; d) all patients with suspected liver disease from other Sections of our Hospital; e) all out-patients who came to our Service during the same period to have their hepatitis markers studied. The highest prevalence of HCV Ab positivity was in the drug users, with a prevalence of 80.9%; far from this value, the next two groups were the patients from the Contagious Disease Section (positivity: 23.4%), and from the other hospital Sections (positivity: 20.1%). In the out-patient group only 9.7% were positive and among blood donors only 0.35%. In all 5 groups the HCV-positive subjects were in many cases positive for B hepatitis too; and very often they presented high levels of ALT. These results confirm that in some the percentage of positive-subjects for C hepatitis or for B & hepatitis; very high; therefore the authors underline the great importance to exclude all members of these groups from the donation of blood, its components, and organs too, even if the tests are negative.