AbstractBy gel exclusion chromatography the molecular weight of T. arcticus LDH was estimated to be 135,000–140,000 and that of MDH to be 65,000–70,000. Employing electrophoretic mobility in starch gel, molecular hybridization, and immunoprecipitation, the LDH isozymes could be classified into three major groups designated a, b and c and a minor group designated β. The c group was found exclusively in the eye. All tissues examined contained a, b and c group isozymes. The interaction of skeletal muscle a‐group isozymes with rabbit anti‐weak fish (Cynoscion regalis) A4 LDH antiserum was weak and, therefore, similar to that of the group of isozymes of Prosobium cylindraceum previously designated f (Massaro, '72). Starch gel electrophoresis at pH 8.6 resolved three major isozymes of supernatant MDH. In skeletal muscle, two additional minor, weakly detectable isozymes were observed. These exhibited a greater net negative charge than the major isozymes and may be conformational isozymes.