Yellow fever (YF) virus is present in patient's blood during the acute phase of illness. Virus isolation and identification provide a potential method of early diagnosis, but available techniques are slow and require specialized materials and equipment. An alternative approach is direct detection of YF antigen in serum by means of an enzyme-linked immunosorbent assay (ELISA). An antigen-capture ELISA was developed, which used anti-YF antibodies, immobilized on a solid phase (polystyrene plates), to capture YF virus from serum samples. After addition of the virus-containing sample, anti-YF detecting antibody conjugated to alkaline phosphatase was added to detect viral antigen. Trials with various capture and detecting antibodies in systems employing purified YF 17D virus, led to the selection of: 1) two capture antibodies (pooled human serum containing high titer YF IgM antibodies and a type-specific YF monoclonal antibody), and 2) a detecting antibody conjugate consisting of monoclonal antibody broadly cross-reactive with all flaviviruses, purified by affinity chromatography, and conjugated to alkaline phosphatase. The limit of sensitivity in tests against purified YF 17D virus diluted in buffer or normal human serum was 103.0–103.6 PFU/0.05 ml or 0.007–0.029 µg viral protein/0.05 ml. Sera obtained at intervals from rhesus and cynomolgus monkeys after infection with a wild YF virus strain were tested. The limit of sensitivity of the assay applied to viremic monkey serum was similar (approximately 3.5 log10PFU/0.05 ml). Monoclonal capture antibody was more potent than human IgM for antigen capture from viremic sera containing high virus titers, whereas IgM provided a more sensitive assay for sera with low titers. Use of monoclonal capture antibody provided a specific virus identification. The procedure offers promise for use as a diagnostic test in human cases.