Urinary lactate dehydrogenase was concentrated and subjected to starchblock electrophoresis. The isoenzyme pattern obtained was shown to be similar to that of the kidney cortex and medulla but different from that of the bladder and kidney pelvis. 2. The values for the relative activity and Michaelis constant of urinary lactate dehydrogenase were similar to those for kidney cortex and medulla but significantly different from those obtained for bladder and kidney pelvis. 3. The molecular weight of urinary lactate dehydrogenase was estimated by thin-layer chromatogtaphy on Sephadex G-200. The values obtained for several samples of urine ranged from 129 000 to 155 000 and were very close to that of the crystalline rabbit-muscle enzyme (140 000). 4. The question of the possible origin of urinary lactate dehydrogenase is discussed and the conclusion drawn that the kidney and not the plasma is the most likely source.
