Abstract: Indirect immunofluorescent antibody test (IFA) using sonicated microfilariae of B. malayi as antigen was carried out to detect filariasis since 1983. The microfilariae were first sonicated into fragments 20-30μm long, and then applied to nohecutane-coated slides. Specific fluorescence was seen at the cut ends of the fragmented microfilariae and along the cuticle. Comparison of results obtained with sera collected by venepuncture and eluates of ear lobe blood on filtet paper showed no statistical difference. Hence, the latter could be used instead of the former.This antigen was tested against 114 plasma samples from both bancroftian and ma-layan microfilaraemia cases and 191 normal controls. At the titre of 1:10, the sensitivity was 86.8% and the specificity, 98.4%.In field investigation, when the same titer was considered as positive, the increase in positive rate among population coincided with the positive rate of microfilaraemia and the variation of positive rates ran parallel with the degree of endemicity. It is concluded that the sonicated microfilarial antigen might be useful for the epidemiological investigation and surveillance of filariasis.