Thesis (Ph.D.)--Boston University / Ehrlich ascites carcinoma lactic acid dehydrogenase was isolated from an eleven-day old tumor by acid precipitation, ammonium sulfate fractionation, and chromatography on DEAE cellulose. Electrophoretic analysis indicated that the final enzyme preparation and the ammonium sulfate fraction contained a single isoenzyme, that is, one of the five possible forms of lactic acid dehydrogenase two of which are tetramers of a single but different protein while the other three are tetramer mixtures of both proteins (i.e. hybrid enzymes). Ultracentrifugal analysis indicated that the final enzyme preparation was composed of two major components with sedimentation rates of 7.3 S and 1.9 S. The enzymatic activity was associated only with the 7.3 S component. The apparent loss of enzymatic activity in 0.1 M phosphate buffer pH 7.0 and the magnitude of the value of the 1.9 S component indicated that this represented the subunit of the enzyme. [TRUNCATED]