The loss of haemolytic activity in sera during storage at low temperature (the cold activation of complement) was observed in 136 of 184 (74%) patients with chronic liver disease associated with hepatitis C virus (HCV) infection. This was more frequent than observed in the three of 40 (8%) patients with chronic hepatitis B (P < 0.001) or none in 43 normal controls (P < 0.001). Of 103 patients with chronic hepatitis C who had completed a full course of recombinant interferon-alpha 2a therapy (total dose: 516 x 10(6) U), 40 responded completely and 21 responded partially, as judged by the normalization or decrease of alanine aminotransferase levels 6 months after the completion of therapy; 42 patients did not respond at all. The cold activation of complement persisted in five (13%) complete responders, less often than in 33 (79%) non-responders (P < 0.001). At the completion of interferon therapy, the cold activation of complement persisted in 12 of 54 patients despite the normalization of alanine aminotransferase. Spontaneous exacerbation of hepatitis occurred in seven of 12 (58%) patients with cold activation, which was more frequent than in the four of 42 patients (10%) without it (P < 0.01). The cold activation of complement disappeared along with the loss of HCV-RNA in five of six responders during the 6 month period after the completion of interferon therapy, while both cold activation and HCV-RNA persisted in all eight non-responders. These results indicate that the cold activation of complement may be useful as a marker of HCV viraemia for monitoring the response to interferon in patients with HCV infection.