The anti-HCV antibody (C100-3) is present in the serum of 70-90% of patients that are carriers of posttransfusion non-A non-B hepatitis. This marker appears to be associated with a viral replication and infectiousness state. Since 1st August 1990 the Swiss Red Cross Transfusion Service has operated a systematic search for anti-HCV antibodies for every blood donation. The aim of the study was to establish the prevalence of anti-HCV antibodies in a donor population, look for the risk factors in the anti-HCV positive group, look for biological symptoms and signs of chronic hepatitis, and compare the data with that from an anti-HCV negative control group. From August to March 1991, 20,373 donors were tested by EIA (Ortho). The presence of anti-HCV antibody was confirmed by a neutralization test (Abbott). The donors in which both tests were positive formed the group studied (55 subjects). Their data was compared with that of a control group of anti-HCV negative donors. The prevalence of anti-HCV antibody in the group of 20,373 donors was 0.29%. Possible parenteral exposure to hepatitis C virus was found in 47% of anti-HCV positive subjects (30% blood transfusion, 9% i.v. drug addiction, 8% tattooing). 42% of the anti-HCV positive donors had no risk factor presently known for hepatitis C. 27% of anti-HCV positive donors had elevated transaminase levels. Until more effective screening tests are introduced it appears necessary to stress the previous history of future blood donors in order to search for hepatitis C risk factors.(ABSTRACT TRUNCATED AT 250 WORDS)