Serologic assay that detects antibodies to Treponema pallidum, the etiologic agent of syphilis. After diluting the patient's serum to remove non-specific antibodies, the serum is mixed on a glass slide with Nichol's strain of Treponema pallidum. An antigen-antibody reaction occurs if the test is positive and the bound antibodies are detected with fluoresceinated antihuman gamma-globulin antibody.