To determine the specificity of the Abbott Murex HIV antigen/antibody combination enzyme immunoassay (EIA) for the diagnosis of HIV infection in Tanzania. A cross-sectional survey of 7333 Tanzanian adolescents and young adults was carried out. Sera testing positive by the Murex assay were further evaluated using a battery of other EIA which detect either antibody to HIV-1 or p24 antigen, and by PCR using pol primers. Of the 674 sera testing positive by the Murex assay, only 53 (7.9%) were confirmed. The specificity of the Murex assay was 91.5%. Serological tests for HIV may perform differently in different populations. New diagnostic tests should not be introduced into populations in which they have not been evaluated.