The aim of the study was to determine the frequency and course of hepatitis C viremia in clinically healthy, anti-HCV positive test subjects, and to ascertain whether the HCV antibodies of the IgM type differed between viremia and immunity. In 21 anti-HCV positive blood donors (test subjects) with normal transaminase activity, two serum samples, taken at an interval of 25 +/- 10 months, have been investigated for HCV-RNA and HCV-IgM antibodies. In a total of 16 test subjects (76%) HCV-RNA was found during the first test and/or the follow-up: 14 of them were positive on both occasions, and one test subject each was HCV-RNA positive exclusively at the first test and the follow-up respectively. At the time of the follow-up the serum transaminase level was elevated in 4 test subjects. 3 of these 4 were HCV-RNA positive also. On the other hand, the results of the HCV-PCR were nonuniform in HCV-IgM antibody negative test subjects. The above results demonstrate that in the majority of clinically healthy, anti-HCV positive test subjects with normal transaminase activity, a viremia exists which persists and the course of which may include inflammatory phases. The proof of HCV-IgM antibodies correlates with a viremia. On the other hand, the lack of HCV-IgM antibodies does not exclude viremia.