Due to the non-availability of sufficient parasite material from Wuchereria bancrofti, a heterologous filarial antigen from Brugia malayi has been investigated for the diagnosis of bancroftian filariasis. B. malayi microfilarial excretory/secretory antigen (BmmfES) effectively inhibited the binding of circulating filarial antigen fractions (CFA2-1,9,11 and 12) from microfilaraemic cases, and of W. bancrofti microfilarial excretory/ secretory antigen, to the immunoglobulin G (IgG) fraction of anti-filarial serum immunoglobulins. BmmfES was separated by ion-exchange chromatography on DEAE cellulose into 2 fractions, BmE DE1 and BmE DE2. BmE DE1 was marginally superior to whole BmmfES and BmE DE2 in detecting filarial IgG antibodies in human sera. 230 human sera from different groups of patients were screened against BmE DE1, which detected specific IgG in 83% of sera from microfilaraemic donors, 83% of sera from patients with clinical filariasis, 17% of sera from normal residents of an endemic area, and in none of the sera from persons living in a non-endemic area. The assay system using BmE DE1, with a sensitivity of 83% and specificity of 83%, should be very useful in detecting microfilaraemia, particularly in active clinical infections where the parasite is usually not seen.