We have tried to define "isozymes" and "differentiation" because these words are often used in a too vague sense. We have noted that the physiological role of isozymes is far from being clearly understood, even for the most studied enzyme with multiple molecular forms, lactic dehydrogenase. But we have pointed out that some isozymes are specific for some adult tissues, especially liver and muscle. They constitute consequently true "markers" of differentiation. Some mechanisms of this differentiation have been made clearer by the experiments of cell hybridization. The role of isozymes in differentiation is also illustrated by the selective disappearance of isozymic markers in some pathologycal conditions, for example : cancer and muscular diseases.