The treponemal group-specific absorbed fluorescent antibody test (FTA-ABS) does not yield clearly positive or negative results in all instances. This study was designed to analyze those sera showing borderline reactivity, in order to determine whether additional tests may help to resolve serodiagnostic problem cases. FTA-ABS tests on 23,807 sera submitted to the Wisconsin State Laboratory of Hygiene yielded borderline results with 479 specimens (2%). Patients showing inconclusive FTA-ABS test reactivity were equally distributed among older and younger age groups, and 6% of borderline cases were women. Of all the 479 specimens, only five sera (1%) gave a non-specific hemagglutination with the Treponema pallidum microhemagglutination (MHA-TP) test, with 82.5% of the FTA-ABS test borderline sera yielding negative results with the MHA-TP test. Because recent findings had shown atypical false fluorescence with the FTA-ABS test on sera from patients with antinuclear antibody, the fluorescent antinuclear antibody test (FANA) was also performed on specimens inconclusive with the FTA-ABS test. Only seven sera (1.5%) were reactive by the FANA test, suggesting that antinuclear antibody was not a significant cause of borderline FTA-ABS reactions. The implications of these findings to venereal disease serology laboratory practice are discussed.