Immunological function in Filipino patients with bancroftian filariasis, manifested as either asymptomatic microfilaraemia or chronic obstructive disease, was compared with that found in healthy control subjects living in the same area. As a group, patients with filariasis had raised serum IgG levels, impairment of antibody responses to tetanus and typhoid vaccines, and suppression of delayed hypersensitivity skin reactions to heterologous antigens. This immunosuppression in filariasis may result from antigenic competition and may contribute to the development of secondary infections.