Biochemical characteristics of middle ear effusions (MEE) should provide a better understanding of the etiopathogenesis of serous otitis media. In order to develop another parameter for the biochemical characterization of the MEE, lactate dehydrogenase (LDH) and LDH isoenzyme patterns in the serous middle ear effusion and serum from 20 patients with serous otitis media were compared. The LDH activity was measured by the usual spectrophotometric method. The isoenzyme patterns were compared on electropherograms using cellulose polyacetate strips. The LDH activity in MEE was significantly higher (P < 0.001) than it was in serum. Fractions of isoenzymes 1 and 2 were each smaller in MEE than in serum. Isoenzymes 4 and 5 have a significantly higher (P < 0.001) fraction in MEE than in serum. Since LDH is an intracellular enzyme and middle ear mucosa is reported to have high content of isoenzymes 4 and 5, the inflammatory changes in the middle ear mucosa which may release intracellular LDH, are suggested as the cause of both higher activity of LDH and the higher fractions of isoenzymes 4 and 5 in MEE than those in the serum.