Specific treponemal antibodies were detected in small amounts by counterimmunoelectrophoresis (CIE) in agar gel, using an extract of T. pallidum. The T. pallidum-CIE test was compared with the fluorescent treponemal antibody-absorption (FTA-ABS) test and with the Reiter protein complement fixation (RPCF) test. The results of the T. pallidum-CIE and FTA-ABS tests agreed in 96% of cases. There was no agreement between the T. pallidum-CIE and RPCF tests in 14% of cases. The 3 tests were compared in 56 sera: 17 from cases of untreated [human] syphilis and 39 from biologically false positive (BFP) reactors. RPCF was positive in 10 (59%) of the 17 specimens from cases of untreated syphilis; all 17 sera were FTA-ABS positive. The T. pallidum-CIE test was positive in 14 (82%) of the 17 specimens. All of the 39 BFP reactors, positive by the RPCF test, were negative according to the FTA-ABS and T. pallidum-CIE tests. The T. pallidum-CIE test is more specific than the RPCF test and slightly less specific than the FTA-ABS test. The advantages of the T. pallidum-CIE test as a convenient method are speed of performance, simplicity and economy.