Summary. This investigation describes the quantitative and qualitative distribution of lactate dehydrogenase (LDH) isoenzymes in subcellular particulate fractions of tissues isolated by differential centrifugation. All fractions, especially those containing microsomes and nuclei, had measurable LDH activity. The supernatant remaining after centrifugation of the microsomal fraction contained 40—50% of the total LDH activity and had an isoenzyme pattern similar to that found in the corresponding tissue homogenate. The cathodically migrating LDH isoenzyme, LDH(5) (M(4)), was found mainly in nuclear fractions and the anodically migrating isoenzyme, LDHX (H(4)), in purified mitochondrial fractions. By mixing in vitro equal amounts of nuclear (LDH(5)) and mitochondrial (LDH(4)) fractions, intermediate isoenzyme patterns were obtained containing LDH(2), LDH(3), and LDH(4). If the nuclear and mitochondrial fractions were mixed in a ratio of 10/1 respectively, then the isoenzyme pattern was similar to that seen in malignant tissue in general. The results suggest that changes in the structural elements of the cell, i.e. nuclei and mitochondria, involves changes in the LDH isoenzyme pattern. According to this hypothesis the LDH pattern of malignant tissue is correlated with well-recognized morphologic changes in malignant cells.