Intracochlear infusion of the KHRI-3 monoclonal antibody results in in vivo binding to guinea pig inner ear supporting cells, loss of hair cells and hearing loss. To further characterize the basis for KHRI-3-induced hearing loss, antibody was produced in a bioreactor in serum-free medium, affinity purified, and compared to conventionally prepared antibody by infusion into the scala tympani using mini-osmotic pumps. In vivo antibody binding was observed in 10 of 11 guinea pigs. A previously unreported pattern of KHRI-3 antibody binding to cells involved in scar formation was noted in five guinea pigs. All but one of the KHRI-3-infused animals demonstrated a hearing loss of > 10 dB in the treated ear. In five of 11 animals the threshold shift was 30 dB or more, and all had hair cell losses. In one guinea pig infused with 2 mg/ml of antibody, the organ of Corti was absent in the basal turn of the infused ear. This ear had a 45-50 dB threshold shift but, curiously, no detectable antibody binding in the residual organ of Corti. Organ of Corti tissue was fragile in antibody-infused ears. Breaks within the outer hair cell region occurred in 5/11 infused ears. The contralateral ears were normal except for one noise-exposed animal that demonstrated hair cell loss in the uninfused ear. Three animals were exposed to 6 kHz noise (108 dB) for 30 min on day 7. Antibody access to the organ of Corti may be increased in animals exposed to noise, since the strongest in vivo binding was observed in noise-exposed animals. Loss of integrity of the organ of Corti seems to be the primary mechanism of inner ear damage by KHRI-3 antibody. The binding of KHRI-3 antibody in new scars suggests a role of the antigen in scar formation. Antibodies with binding properties similar to KHRI-3 have been detected in 51% of patients diagnosed with autoimmune sensorineural hearing loss; thus, it seems likely that such autoantibodies also may have pathologic effects resulting in hearing loss in humans.