Sera from patients with various clinical pictures of lymphatic filariasis, including tropical pulmonary eosinophilia (TPE), were used for the localization of surface and intracellular antigens in microfilariae of Wuchereria bancrofti embedded in Lowicryl K4M. Very few or no antigenic sites were located on the outer face of the sheath. The most inner layer, as well as the space between the cuticle and the sheath, was intensely labeled. Sera from TPE patients intensely labeled the cuticle and the cytoplasm of muscle cells.