We investigated three immunoblot assays (RIBA 3.0 from Chiron, Matrix from Abbott Laboratories, and LiaTek III from Organon Teknika) for the detection of antibody to hepatitis C virus. RIBA 3.0 and Matrix require reactivity to two antigens and LiaTek III requires reactivity to one for a sample to be positive. We tested 80 samples that were positive in repeat enzyme immunoassays by supplemental tests. The results showed that 55, 46, and 28% were reactive by LiaTek III, RIBA 3.0, and Matrix, respectively; 54, 33, and 13% of the samples were indeterminate by Matrix, RIBA 3.0, and LiaTek III, respectively. There were 32, 21, and 16% nonreactive samples by LiaTek III, RIBA 3.0, and Matrix, respectively. Of the samples positive by RIBA 3.0, only 50 and 76% were reactive by Matrix and LiaTek III, respectively. A large number of samples that were indeterminate by RIBA 3.0 were positive by LiaTek III (52%). The core antigen was the most reactive antigen in all three tests (48 to 57%). The NS4 antigen in Matrix (20%) and LiaTek III (16%) was poorly reactive, although it performed better in RIBA 3.0 (45%). The NS5 and E2/NS1 antigens made minor contributions to reactivity. The combinations of the core, NS3, and NS4 antigens produced 77% of the RIBA 3.0 and 100% of the Matrix reactive samples. The results showed a poor correlation among the three tests.