To assess the sensitivity and specificity of a newly-developed assay (Bioelisa HIV-1 + 2, HTLV-1 + 2) for the simultaneous detection of HIV-1, HIV-2, HTLV-I and HTLV-II antibodies in human serum or plasma specimens.A panel of 775 well characterized serum or plasma samples was studied. This included samples confirmed to contain antibodies to HIV-1 (n = 46), HIV-2 (n = 19), HTLV-I (n = 49) and HTLV-II (n = 12), samples containing low titres of anti-HIV antibody (n = 14) and samples collected during HIV seroconversion (n = 36). Eighty-three sera samples which were reactive in one or more HIV or HTLV screening assays, but which could not be confirmed to contain anti-HIV-1/2 or anti-HTLV-I/II antibodies, were also examined.Excluding the seroconversion samples and those selected on the basis of false reactivity in other screening assays, the Bioelisa kit had a sensitivity of 100% for antibody to all four viruses and a specificity of 98.8%. The ability of the kit to detect anti-HIV during seroconversion was similar to that of several other synthetic HIV-antigen-based screening kits currently in use.Our findings indicate that the Bioelisa kit is sufficiently accurate to screen for both HIV and HTLV infections and that it warrants larger scale trials. Its use might allow blood donor screening for HTLV infection to be introduced more widely at modest extra cost [corrected].