The presence of circulating hepatitis C virus genome (HCV-RNA), elevated ALT levels and antibodies to an NS5-derived synthetic peptide have been examined in 13 subjects with isolate positivity for antibodies to the HCV core antigen (C22) on RIBA-2 testing. All subjects were followed up for 8-18 months (mean 12.4 months). In seven subjects (54%), intermittent or persistent viremia was associated with abnormal ALT levels (6 subjects) and with positivity for antibodies to NS5-peptide (6 subjects). On the other hand, in 6 out of 13 subjects (46%) no viral replication, no liver cytonecrosis and no antibodies to NS5 were found. It is concluded that isolate reactivity to C22 by RIBA-2 is a heterogeneous condition that corresponds to two distinct categories of subjects: those with active HCV infection and those without evidence of virus replication. Although HCV-RNA determination is the most reliable means of identifying HCV carriers, antibodies to NS5 can be a useful marker of virus activity. In fact, antibodies to NS5 were detected in 6 out of 7 viremic patients, compared to 0 out of 6 non-viremic patients (P = 0.004). It remains to be elucidated whether the isolate reactivity to core antigen found in non-viremic subjects represents a specific, HCV-induced antibody response, or is an unrelated crossreactivity.