The recently developed O g 4 c 3 ELISA, which detects circulating Wuchereria bancrofti antigen, appears promising for use in epidemiological surveys, but its sensitivity is unknown in persons with ultra-low microfilarial densities. We used the O g q c 3 to test the sera of 282 persons who were microfilaria-positive in 1-16 ml of blood, 18 persons who were microfilaria-negative but who had ultrasonographic or biopsy evidence of adult W. bancrofti infection, and 63 lifelong residents of a non-endemic area of Brazil. A total of 276 (97.9%) persons with detectable microfilaraemia tested positive (optical density >0.03 3). At microfilarial densities of 30 microfilariae per ml of blood, the sensitivity of the O g q c 3 was 72.2, 97.6 and IOO%, respectively (X'-test for trend, PcIo-'). The assay was positive in 66.7% of amicrofilaraemic persons with evidence of adult worm infection and in one (1.6%) of 63 residents of the non-endemic area (specificity, 98.4%). Our findings support the increasingly widespread use of the OgqC3 for field investigations and epidemiological assessments. However, the sensitivity of the assay may be low in persons who are microfilaria-negative or with densities of < I microfilaria per ml.