1. Preliminary results on the biochemical analyses of middle ear effusions both in human and in experimental animals are reported. 2. A higher concentration of potassium as well as total protein increase compared to serum was observed in human serous middle ear effusions. In animal studies, no difference of total protein concentration between effusion and serum was observed suggesting this fluid is a transudate from vessels. 3. Both in human and experimental animals, the pattern of protein components on electrophonegram resembled closely that of serum although in both a slight elevation of beta globulins was observed in middle ear effusions compared to serum. 4. Higher concentrations of immunoglobulins (IgG, IgA) in human middle ear effusions as well as somewhat altered ratio (IgGjIgA) compared to serum were observed. 5. Lower glucose concentration was observed in human middle ear effusions compared to serum. 6. Both oxidative (LDH, MDH) and hydrolytic (alkaline and acid phosphatase) enzymes were present as higher concentration in the human middle ear effusion than in serum. 7. Possible mechanisms for differences between the middle ear effusion and serum were discussed.