Specimens were heated at 56 degrees C or 62 degrees C to analyze the mechanisms of false positive reactions in the Ortho enzyme-linked immunosorbent assay (ELISA) for the detection of antibody to hepatitis C virus (HCV). The highest correlation was obtained between the difference in HCV antibody titer of 56 degrees C-heated and native sera and IgG concentration. Monoclonal IgG gammopathy sera developed marked increases in the titer by preheating. However, slight increases after preheating were observed in monoclonal IgA gammopathy sera. Similar increases in the titer by preheating were also demonstrated in immunoglobulin products containing IgG. Direct bindings of denatured IgG with C100-3 and 5-1-1 antigens were shown in recombinant immunoblot assay (RIBA). These results could show the participation of denatured IgG in the false positive reaction in Ortho-HCV-ELISA. Caution is necessary in evaluating the anti-HCV antibody titers of heat-inactivated sera.