The counter-immunoelectrophoresis technique with the Reiter protein (RP-CIE) was compared with two complement fixation tests (Kolmer and RPCF) and a flocculation test (VDRL) in sensitivity and specificity. Of the 1,927 consecutive attendants of a venereal disease clinic whose serum samples were used, 250 were considered to be syphilitic. The number of true-positive and false-positive reactions were: 121 and 4 (VDRL), 124 and 2 (Kolmer), 179 and 41 (RPCF), 166 and 16 (RP-CIE). The VDRL and the RPCF combined were more sensitive and less specific than the VDRL combined with the Kolmer. If the RPCF was replaced by the RP-CIE the sensitivity remained the same but the specificity was higher. The RP-CIE scored more positives than the RPCF in untreated cases of primary syphilis. The results argue for substitution of the RP-CIE for the Kolmer in the combination with the VDRL in the serodiagnosis for syphilis. Moreover, the RP-CIE presents the technical advantages of simplicity, speed of performance, and of not being hampered by the anticomplementary nature of the serum sample.