Although the HIV-1 group O virus found in two persons of Cameroonian origin has been described in 1990 (De Leys et al., 1990), sera from group O infected individuals became available only recently. Several studies showed that some of the anti-HIV-1/HIV-2 screening tests failed to detect all of these samples (Loussert-Ajaka et al., 1994; Simon et al., 1994; Schable et al., 1994; Gürtler et al., 1995). In the current version of an anti-HIV-1/anti-HIV-2 screening assay, namely the Vironostika HIV Uni-Form II, an HIV-O specific peptide was introduced in order to improve HIV-1 group O reactivity. The peptide was derived from the immunodominant region of HIV-1 group O gp41 strain ANT70. All 30 anti-HIV-1 group O sera were detected by the so-called plus O assay, while 29 samples of this panel were positive the current assay. The sensitivity of the plus O assay for anti-HIV-1 and anti-HIV-2 positive samples is identical to that of the reference test without HIV-1 group O peptide. The clinical specificity of the HIV Uni-Form II plus O assay is improved to > or = 99.92% by an adjustment of the coat concentration of HIV-1 p24 (to avoid false positive p24 only reactions) without affecting sensitivity of the assay. The specific reaction of an HIV-1 group O specific rabbit serum for quality control purposes is presented.