The round window membrane is the only soft tissue barrier between middle ear and inner ear and the main route of access for potentially ototoxic substances from middle ear cavity into inner ear. Since the round window membrane is biologic membrane, there has to be selectivity involved. There are factors such as the molecular weight of the substances, the nature and concentration of the substances in the middle ear cavity and the presence and duration of the inflammation in the middle ear, etc that influence the passage of the substance through the round window membrane. Cefotiam hydrochloride is cephalosporin antibiotic and low molecular weight substance(MW 598.5). By application of cefotiam hydrochloride on the round window membrane, the following results were obtained ; 1) We could detect the cefotiam hydrochloride in inner ear fluid and it indicate that round window membrane is permeable to cefotiam hydrochloride. 2) The concentration of cefotiam hydrochloride in inner ear fluid increased, as application time of cefotiam hydrochloride become longer. 3) The concentration of cefotiam hydrochloride in inner ear fluid was maintained so sufficiently as to inhibit the growth of the organisms susceptible to cefotiam hydrochloride.