Lymphocyte proliferative responses to homologous and heterologous filarial antigens, mitogens and purified protein derivatives (PPD) were analysed in a group of 37 subjects from an area endemic for bancroftian filarial infection. The majority of the subjects without any clinical or parasitological evidence of filariasis (endemic normals) reacted with homologous microfilarial antigens only. Non-treated patients with patent microfilaraemia, did not respond to homologous of heterologous microfilarial antigens. In contrast, diethylcarbamazine (DEC)-treated microfilaraemic patients, reacted with homologous filarial antigens. Patients with elephantiasis reacted to microfilarial and adult worm antigens. Response to PPD was marginally depressed in patent microfilaraemic patients and a rise was observed in elephantiasis cases. Endemic normals exhibited normal response to PPD. Responses to mitogens were depressed throughout the course of the infection.