Some immunosuppressed patients with hepatitis C virus infection do not have detectable levels of antibody to hepatitis C virus on second-generation enzyme immunoassay. Antibodies to the envelope and nonstructural region 5 proteins have not been examined. Four groups of patients with hepatitis C virus infection were studied: (a) 20 immunocompetent patients, (b) 15 hemodialysis patients, (c) 17 kidney transplant recipients and (d) 3 acute leukemia patients who underwent bone marrow transplantation. Serum samples were tested for antibody to hepatitis C virus with a second-generation enzyme immunoassay and multi-antigen enzyme immunoassays and for hepatitis C virus RNA with a nested polymerase chain reaction assay. All the immunocompetent patients reacted to C25, C22 and C33C; 90% reacted to nonstructural region 5 antigen and 80% reacted to C100-3. Only 55% reacted against yeast-derived e1 and e2 antigens, but all reacted against vaccinia virus--expressed N e1 and e2 antigens, indicating that the envelope epitopes are conformational and glycosylated. Sixty-five percent to 90% of dialysis and kidney transplant patients reacted to C25, C22 and N e1 and e2, but only 12% to 60% reacted to C100-3, C33C and nonstructural region 5 antigen. Diminution or loss of reactivity to hepatitis C virus antigens was observed after kidney and bone marrow transplantation, with C25 and N e1 and e2 less affected. Our data suggest that incorporation of C25 and N e1 and e2 antigens in the assay for antibody to hepatitis C virus would improve the detection of hepatitis C virus infection in immunosuppressed patients.