A study was undertaken to define the sensitivity and specificity of serological tests in bancroftian and malayan filariasis and correlate the findings with clinical disease. Sera were collected from subjects on three different islands in the Philippines: one endemic for bancroftian filariasis, another endemic for malayan filariasis and the third without endemic filariasis. Antibodies were measured, using Brugia malayi as the source of antigen. Antibodies against adult worms measured by indirect immunofluorescence were found at a titer of 1:8 or greater in all patients with bancroftian or malayan filariasis but not in the control subjects. There was no relationship of antibody titer to clinical status. Antibodies against microfilariae were measured by indirect immunofluorescence and by microfilarial agglutination. A high correlation was observed between the two methods. These antibodies were found in only one quarter, approximately, of patients with filariasis. Microfilarial antibodies were found more commonly in those patients with chronic lymphatic obstruction. It is concluded that measurement of antibodies to adult worms is a useful indicator of infection while microfilarial antibodies are correlated with disease.