A technique has been developed, based on preferential inhibition by urea, for determining the amounts and proportions of the M and H sub-units of lactate dehydrogenase (referred to as LDH-M and LDH-H respectively) in human tissues, including muscle. 2. There was good agreement between the results obtained with urea inhibition and those obtained with starch-gel electrophoresis. 3. With increasing age there was a significant decrease in the total amount of lactate dehydrogenase and the amount of LDH-M in skeletal muscle. This could not be accounted for by the replacement of functioning muscle tissue by fibrous connective tissue. 4. The proportion of LDH-M was less in certain muscles (e.g. soleus and extra-ocular) than in other muscles (e.g. gastrocnemius and rectus abdominis). 5. The proportions of LDH-M and LDH-H did not differ significantly in different superficial limb muscles and were not significantly affected by either age or sex. 6. Specimens of muscle from 86 different individuals (all Europeans) have been subjected to electrophoresis, but no variants of lactate dehydrogenase isoenzymes have been found.
