The possibility of developing a satisfactory serological test applicable to seroepidemiological studies of brugian and bancroftian filariasis was investigated. The cat-Brugla uahungi model was used. The relative sensitivities of the indirect fluorescent antibody test (IFAT), counter- immunoclectrophoresis (CISP) and ensyme-linked immunosorbent assay (ELISA), using crude or purified somatic, and metabolic antigens of various developmental stages of six different human and animal filarioids, for the detection of antibodies in sera were compared. Homologous antigens were always better than heterologous antigens in sensitivity. Metabolic antigens seemed to be more species-specific. A rise in antibody level was observed after chemotherapy. IFAT using B. pahanrj and Wuchereria bancroiti infective larvae as antigens were the most sensitive tests for feline (89% positivity), and human brugian (96%) and bancroftian (100%) filariasis respectively. In ELISA, fractions 1 and 2 of the partially purified antigen of B. nahangi adults seemed to be more specific for bancroftian and brugian filariasis respectively than the crude antigen. ELISA and IFAT were found useful in the seroepidemiological studies of human filariasis in New Guinea, Trinidad, the Seychelles Islands and Egypt. ELISA and CIEP wore used to detect circulating worm antigen in sera and urine, using rabbit antisera to B. pahangi antigen. Studies with phosphoglucomutese.. isoenzymes of B. pahangi adult worms suggested that these isoenzymes could be the source of specific antigen for serological tests. The histopathology of the lungs, liver, spleen, kidneys, infected lymph nodes and lymphatics of B. pahangi-infected cats was studied. A higher percentage of glomeruli in kidneys from 71 infected cats showed histological lesions when compared to the uninfected controls. Fluorescence studies demonstrated the presence of IgG» worm antigens and complement deposits in the glomeruli of kidneys from infected cats. 40% of infected cats had proteinuria by Albustix test. The presence of circulating immune complexes in sera from infected cats and humans with brugian and bancroftian filariasis was studied by C1q- and conglutinin-binding assays.