Determination of lactic dehydrogenase (LDH) activity has been shown to have diagnostic value in pathologic states involving tissue necrosis and neoplasia. Necrotizing tissues release this enzyme, which catalyzes the reduction of pyruvate to lactate or the reverse, in an active state. As a result of this release, the enzyme appears in body fluids, notably the blood. Increase of this activity over normal indicates necrosis. The present paper compares the results of the Wroblewski and LaDue and the Wacker et al., methods for measuring LDH activity. Results of the two methods were found to be not comparable. No calculation can be found to relate the results of the two analytical methods. Only 18 of 30 samples gave abnormally high results by both methods. Changes between serial samples drawn on the same patient are not always in the same direction when measured by both methods. Lack of mathematical correlation between the kinetics of the pyruvate-to-lactate reaction and the lactate-to-pyruvate reaction is assumed to explain this.