The molecularheterogeneity of many proteins, particularly of individual enzymes, has recently come to be clearly recognized. The various forms of these proteins are usually separable by physi-cal means, but the physiological sig-nificance of the separate molecular species has frequently been obscure. Regarding theenzyme lactic dehydro-genase (LDH), however, the different molecular forms appear to have differ-ent functions and it has become clear that they are associated with different modes of metabolism and are quite characteristic of specific tissues. The molecular forms of LDH were first separated by electrophoresis; by this means up to five separate forms can be distinguished. Recent studies have demonstrated that in fact only two principal or parent forms of LDH exist. The present view, now supported by evidence from many sources, is that