Middle ear fluids from 123 patients with serous otitis media were analysed by comparing elements of the fluid with the patients' sera. Results of paper electrophoresis, immunoelectrophoresis, and disc electrophoresis showed no qualitative difference between serum and middle ear fluid. Most fluids further contained antibodies to rabbit erythrocytes, and the agglutination titers were remarkably similar to those in the corresponding sera. Even γM antibodies were present in serum concentrations. An antiserum produced by immunization of a rabbit with a pool of ear fluid behaved as an antiserum to whole human serum. No additional lines appeared in tests with ear fluid. The fluid is probably a transudate of plasma, produced as a result of a negative pressure in the middle ear due to a dysfunction of the eustachian tube.