Multiple molecular forms of lactate dehydrogenase fractionated by isoelectric focusing with ampholines in the polyacrylamide gel thin layer were identified in the supernatant fluid of the myocardium and liver homogenate of the intact rats and rabbits. Differential detection of the enzyme fractions was based on differences in their substrate dependences and sensitivity to specific inhibitors. Simultaneously the analysis was made for the partially purified preparations of lactate dehydrogenase five isoenzymes from the same sources obtained by means of the preparative electrophoresis in polyacrylamide gel and ion-exchange adsorption on DEAE-Sephadex A-50. The isoelectric points of the identified fractions are determined. It is shown that redistribution of the enzyme multiple molecular forms within the pH gradient is subordinated to a certain strictly regularity: fractions from LDH1 to LDH5 occupy successively the sites in a weak-acid, neutral and weak-alkaline regions of the pH gradient.