The unheated serum reagin (USR) test, a standard qualitative test for syphilis, was evaluated for use as a quantitative test for the serodiagnosis of syphilis and as a means of determining the effectiveness of treatment. Tests were performed on 664 syphilitic sera from treated and untreated patients in the primary, secondary, and latent stages of the disease. Also included were 95 false-positive sera and 36 sera from presumably normal donors. The Venereal Disease Research Laboratory slide and rapid plasma reagin 18-mm circle card tests were used as a basis for comparison with the USR test. Results of the USR quantitative test paralleled the Venereal Disease Research Laboratory and rapid plasma reagin card quantitative tests with approximately 84% agreement. The advantages of the USR test make the findings of this study significant in that: (i) less technical manipulation is required to perform the USR test than the Venereal Disease Research Laboratory slide test; (ii) no labor cost is incurred for daily Venereal Disease Research Laboratory antigen preparation; and (iii) the USR antigen is less expensive than the rapid plasma reagin card tests antigen.