11 acoustic neuroma patients, exhibiting cell-mediated immunity in vitro against acoustic neuroma extract, were tested for cell-mediated immunity in vitro against perilymph samples from other acoustic neuroma patients. 14 of 21 perilymph samples were antigenic. None of three healthy persons reacted against antigenic perilymph samples. The perilymph antigenicity was reproducible and negatively correlated to the perilymph sample volume. Differences in sample volume could not be ascribed to admixture of endolymph, cerebrospinal fluid, plasma or blood. The perilymph protein concentration varied from 31 to 54 g/l. Gel electrophoresis of perilymph proteins revealed less staining in the alpha-region compared to plasma proteins and a distinct band in the pre-gamma region absent in plasma. Tau-transferrin was not detected in any of 13 samples. The findings support that the cell-mediated immune response against acoustic neuroma extract may be mediated through release of antigen(s) to the perilymph.