We previously described a Sendai virus (SeV)-based expression system for the recombinant gp120 of HIV-1 subtype B (rgp120-B), which has permitted the production of antigenetically and functionally authentic gp120 at a concentration as high as 6 microg/ml of culture supernatant (Yu D et al.: Genes Cells 1997;2:457-466). Here the same procedure was successfully applied to the production of HIV-1 subtype E gp120 (rgp120-E). The remarkable production of the proteins by the SeV expression system enabled us to use crude culture supernatants for serological and functional studies of gp120s. The immunological authenticity of rgp120-E was verified by patient sera and anti-V3 loop monoclonal antibodies specific for HIV-1 subtypes B and E. CD4-binding properties were corroborated by FACS analyses. The rgp120s were then used in an enzyme immunoassay (rgp120-EIA) to detect antibodies in the sera of HIV-1-infected individuals, and the performance was assessed in comparison with a conventional V3 loop peptide EIA (V3-EIA). The initial evaluation of a serum panel (n = 164) consisting of 76 subtype E and 88 subtype B sera revealed that the rgp120-EIA was nearly 1000-fold more sensitive than the V3-EIA and was able to detect subtype-specific antibody with 100% sensitivity and with a complete correlation with the genotypes, whereas the V3-EIA failed to detect 9 and 24% of the same subtype E and B sera, respectively. Furthermore, a study employing a panel of 28 international sera with known genotypes (HIV-1 subtypes A through F) confirmed the remarkable specificity of this method. An EIA reactivity higher than 1.0 was an unambiguous predictor of HIV-1 subtype E and B infections. The data imply the presence of strong subtype-specific epitopes for antibody bindings to these rgp120s.