Sera from a small percentage of hepatitis C virus (HCV)-infected blood donors do not react in the currently available assays for detection of antibody to HCV (anti-HCV) and, as a consequence, hepatitis C may develop in recipients of this blood. One possible explanation for this phenomenon is that antibody is present but cannot be detected because it is sequestered in circulating immune complexes. To test this hypothesis, an immune complex dissociation (ICD) assay was developed to disrupt any immune complexes that might be present in these anti-HCV-negative, HCV RNA-positive sera. A positive result in this test would indicate that antibody is present in these patients but is not detectable under routine anti-HCV testing conditions. Nine chronic and two acute HCV patients, all negative for antibody but positive for HCV RNA by reverse transcriptase-polymerase chain reaction (RT-PCR) were tested, together with appropriate controls. Three of the nine study patients with chronic HCV had evidence of anti-HCV after immune complex dissociation compared with none of the two patients with acute HCV. Although the number of patients tested was small, the negative results in the patients with acute HCV presumably indicates that anti-HCV seroconversion had not yet occurred. Incorporation of an ICD step into existing anti-HCV assays may enable blood banks to detect those rare instances of patients with chronic HCV who are antibody negative; this would minimize potential cases of post-transfusion hepatitis in recipients.