The serodiagnosis of human immunodeficiency virus (HIV) infection has widely been established by the screening test and the confirmatory test. At present, Western blot (WB) assay is mostly used as the confirmatory test. However, this method has the problem in that the sensitivity and the specificity are not enough. A new confirmatory test "CHIRON RIBA HIV-1/HIV-2 SIA" developed by Chiron Corporation uses an immunoblot enzyme immunoassay technique for detection of anti HIV-1 and/or HIV-2 antibodies. This assay employs four recombinant viral antigens (gp120, gp41, p24/p26 and p31) and a synthetic viral antigen (HIV-2 envelope peptide). The characteristic of this method is that the HIV-1 infection and the HIV-2 infection can be differentiated from each other. We therefore compared this SIA method with the WB1 assay for detection of anti HIV-1 antibodies and with the WB2 assay for detection of anti HIV-2 antibodies. Eighty samples from normal adults without HIV infection and known to be negative by three HIV screening tests, respectively, were tested by SIA, WB1 and WB2 assays. The negative rates (specificities) were 97.5%, 80.0% and 87.5% by the SIA, WB1 assay and WB2 assay, respectively. With forty samples from patients without HIV infection but known to be positive by at least one HIV screening test, the negative rates (specificities) were 97.5%, 72.5% and 85.5% by the SIA, WB1 assay and WB2 assay, respectively. The results indicated that the SIA method was more specific than two WB assays. Forty samples from patients with HIV-1 infection and known to be positive by three HIV screening tests, were tested by the SIA and WB1 assay. The positive rates (sensitivities) were 97.5% and 75.0% by the SIA and WB1 assay, respectively. With thirteen samples from patients with HIV-2 infection and known to be positive by three HIV screening test, the positive rates (sensitivities) were 100% and 92.3% by the SIA and WB1 assay, respectively. The results indicated that the SIA method was more sensitive than the WB1 assay. Three sets of sera, which were collected during seroconversion for HIV-1 antibody, were used to compare the positive readings by the SIA and WB1 assay. The SIA method indicated the positive readings earlier than the WB1 assay. The present findings indicated that the SIA method was more specific and sensitive than the WB assay, and would be useful as a confirmatory test.