An enzyme immunoassay for the diagnosis of syphilis (ELISA-SY) was developed with solid-phase extracts of Treponema pallidum, specimen diluent containing Reiter treponeme absorbent, and three 30-min incubations. The ELISA-SY results were determined in comparison with a standardized positive control and reported as a percentage of strong positive control. In tests with 1,005 serum samples from a venereal disease clinic and other sources, 98.2% agreement was found with fluorescent treponemal antibody-absorption (FTA-ABS) results, and 98.3% agreement was found with T. pallidum passive hemagglutination (PHA) findings. Only 1 of 29 sera originally considered to be biologically false-positive by ELISA-SY; the latter specimen was also positive by PHA and FTA-ABS tests performed in our laboratories. Serum samples from clinically diagnosed syphilitics (16 primary-stage isolates, 7 secondary-stage isolates, and 3-latent-stage isolates) were all positive by ELISA-SY, FTA-ABS, and PHA. Serum samples from 51 newborns suspected of having syphilis on the basis of positive cardiolipin flocculation tests showed 98% agreement of ELISA-SY results with FTA-ABS and PHA findings. Sera from all 61 patients with a variety of autoimmune and other diseases known to be associated with biologically false-positive reactions for syphilis were negative by this ELISA-SY. The specificity of the ELISA procedure for T. pallidum antibody was also confirmed immunologically by blocking experiments.