A recent theory, suggesting that otosclerosis results from autoreactivity to type II collagen present in the fetal cartilaginous remnants of the human bony labyrinth, is based on two observations. Otosclerotic patients have increased concentrations of circulating antibody to type II collagen, and immunization of rodents with cartilage collagen induces 'otosclerosis-like' lesions. Independent researchers have been unable to confirm the first promising results. No significant abnormalities could be found in immunized animals. We report the result of type II collagen antibody recordings in a well described group of otosclerotic patients and controls, and the histological findings in temporal bones of MRL/1-mice with spontaneous type II collagen autoreactivity. Our results cannot support the view of autoreactivity to type II collagen as an etiopathogenetic factor in otosclerosis.