Experimental allergic neuritis (EAN) was induced in guinea-pigs by intradermal injection of rabbit peripheral nerve emulsified in Freund's adjuvant. Both sciatic nerves were obtained between 12--24 hr after clinical symptoms were evident. Several fascicles from each nerve were isolated for histochemical studies with NADH-diaphorase (NADH) and acid phosphatase (AP) applied to teased nerve fibres. Small pieces were processed for electron microscopy, and a fascicle was teased after staining with osmium tetroxide. In isolated nerve fibres stained with histochemical techniques myelin lesions of segmental character were found closely related to inflammatory cells; Schwann cell cytoplasm in contact with mononuclear cells showing a heavy enzymatic activity (NADH and AP). Under polarized light, the underliying myelin showed focal loss of birefringence. Some electron-microscopic pictures suggested active myelin breakdown by mononuclear cells. The possibility of primary Schwann cell damage by mononuclear cells with subsequent demyelination is discussed.