The results of experimental investigations on the pharmacokinetics of mezlocillin in the perilymph after i.m. injection of 200 mg per kg in the guinea pig are represented graphically. A retention of this new acylureidopenicillin in the inner ear, similar to that of the aminoglycoside antibiotics, is observed. The consideration of the mezlocillin perilymph concentrations that resemble those of gentamicin on a weight for weight basis and the clinically equivalent dosages of these two antibiotics show clearly that the perilymph concentrations of mezlocillin, that may be clinically reached, are many times higher than those of gentamicin. Finally, the prophylactic and therapeutic importance of the mezlocillin perilymph concentrations for infectious inner ear complications caused by the usual bacteria of the acute otitis media or the problem germs of chronic otitis media is discussed.