By using an antigen-induced (Arthus) otitis media (OM) model in chinchillas sensitized with human serum albumin (HSA), we investigated the passage of HSA through the round window membrane (RWM) by measuring HSA levels in both the middle ear fluid (MEF) and the perilymph (PL). The effect of corticosteroid treatment on HSA passage was also studied. Mean HSA levels in both the MEF and the PL were proportional to the HSA dose administered (0.5, 5, 50 mg/ml). The proportion of HSA-positive PL samples was significantly lower in the sensitized group than that of the control (p<0.01). HSA levels in the PL appear to depend on HSA levels in the MEF, which were significantly depressed in the sensitized animals. These results suggest that an immunological reaction may be involved in the reduction of antigen (HSA) passage through the RWM by reducing antigen concentrations in the MEF. Corticosteroid treatment reduces the passage of HSA into the PL in non-sensitized animals.