Summary. Ninety consecutive patients with chronic hepatitis C were included in a randomized, uncontrolled trial to compare the efficacy of two treatment regimens, 10 MU (group A) vs 5 MU (group B), of lymphoblastoid interferon, in a step-down schedule for 24 weeks. All of the patients had antibodies against the hepatitis C virus, and all but one were HCV RNA positive in serum. The origin of the infection was attributed to blood transfusion in 30 patients and classified as sporadic in 60 patients. During treatment reduction in the ALT levels as well as the elimination of viraemia was observed in both treated groups, although these changes did not correlate significantly with the interferon dose. Nine months after the end of therapy, a sustained response was achieved in 13.6% (12/88) of the patients. Relapse in group B (87.5%) was significantly higher than in group A (59.1%). The percentage of cases which remained with undetectable HCV RNA was significantly higher for the sustained responders (66.7%) than for the non-responders (11.8%) and relapser patients (2.4%). Repeated liver biopsies showed an overall significant reduction of all the subindices of histological activity from patients with sustained response, except for fibrosis. In short: the 10 MU dosing regimen of lymphoblastoid interferon was as efficient as the 5 MU dose as it brought about a similar improvement in ALT levels, histological activity and elimination of viraemia, albeit 10 MU proved significantly more effective in the prevention of a relapse among the responders after 24 weeks therapy.