Indirect fluorescent antibody test (IFAT) using frozen sections of Brugia malayi adult worms as antigen was employed in the diagnosis and epidemiological investigation of human filariasis. Sera were collected from 704 cases with bancroftian or malayan microfilaremia. the positive rate was 92.8-99.1%. Of 150 healthy people from non-endemic areas, only one showed a positive reaction (false positive rate 0.7%) (Table 1). This technique proved to be highly sensitive and specific for the diagnosis of filariasis and the antigen is easy to prepare. It might be used in sero-epidemiological investigation for the assessment of filariasis control.