Malic dehydrogenese (MDH) activities in human serum of various diseases were studied. Elevation of MDH activities were observed in myocardial infarction, acute hepatitis, and some malignant tumors. Characteristic MDH isozyme patterns were obtained from serum of patients with myocardial infarction and acute hepatitis showing marked elevation of MDH activities. During the tetrazolium procedure for staining of electrophoreticaly separated serum malic or lactic dehydrogenese isozymes on agargel, without substrate and co-enzyme, two peaks were noticed and named serum "non-specific factor". These peaks were considered a kind of non specific reactions which resulted from a reduction of the tetrazolium salt electrostaticaly adsorbed on Alb. ～ α(1), Glb. by SH-groups of these serum proteins. These reactions were accelerated by the elavation of PH or the temperature, exposure during staining and the prolonged staining time, as well as the rise of concentrations of NTB, PMS and CN- in the staining medium. Considering of optimal conditions of enzyme reaction, the minimum use of NTB, PMS and Na CN, washing stained agargels with PH 4.5 acetic acid SoL, and complete shading during procedure could minimize the influence of this factor. Two MDH isozymes were demonstrated in agargel isozymograms of rat organs, one migrating towards the cathod and the other toward the anode. The cathodal fraction was considered mitochondrial MDH (m-MDH), and the anodal one cytoplasmic MDH (c-MDH), respectively. During development, m-MDH showed marked increase in heart muscle. In contrast to this, cMDH of liver and gastric mucosa increased in procedure of development. On the other hand the ratio of m-MDH and c-MDH in kidney remained constant through development.