Circulating filarial antigen (CFA) isolated from the plasma of microfilaraemic patients was fractionated on an Ultrogel ACA 34 column. The second protein peak (CFA2) showing filarial antigen was further fractionated by DEAE-cellulose column chromatography into two fractions (CFA2 DE1 and DE2). CFA2 DE1 fraction, showing antigenic activity, was further evaluated in an ELISA for its diagnostic use in bancroftian filariasis. Studies with CFA2 DE1 and anti-CFA2 DE1 antibody showed that they were highly active in the detection of filarial antibody and antigen in asymptomatic microfilaraemia sera and thus obviate the need for the tedious night blood collection and examination. Fractionated filarial plasma can be another candidate antigen for immunodiagnosis of bancroftian filariasis.