Humoral immune responses of the Indian leaf monkey (Presbytis entellus) experimentally infected with Brugia malayi and exhibiting disease manifestations were studied. Microfilaraemia, filaria-specific IgG and circulating immune complexes (CICs) were determined in the monkeys at different time-points after inoculation of B. malayi 3rd-stage larvae. Sera were analysed for recognition pattern of adult parasite antigen molecules by immunoblotting. More than 60% of the infected monkeys developed episodic or persistent limb oedema with or without fever and with low or no microfilaraemia. While both CIC and filaria specific IgG levels were comparable in animals showing no disease symptoms (asymptomatics) and some animals showing symptoms (symptomatics), IgG levels peaked during pre-patent stage in symptomatics and during latent stage in asymptomatic animals. However, some of the symptomatic animals showed a low level of filaria-specific IgG as compared to asymptomatic and other symptomatic animals. The immunoblot analysis showed non-reactivity of 17 and 55 kDa antigens with sera of symptomatic animals. The results thus suggest that humoral immune responses as measured in the present study do not precede the development of the manifestations. However, 2 non-reactive antigen molecules identified by symptomatic sera need further study to establish their possible involvement, if any, in the development of acute disease manifestations in this model.