The recently developed Og4C3 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 Og4C3 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 >0·033). At microfilarial densities of <1, 1–30, and >30 microfilariae per ml of blood, the sensitivity of the Og4C3 was 72·2, 97·6 and 100%, respectively (χ2-test for trend, P<10-6). 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 Og4C3 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 <1 microfilaria per ml.