Urinary and blood isoenzymes and other biochemical tests were performed on fresh or fresh-frozen specimens from intact renal-allografted or renal-autografted adult male and female Chacma baboons. Although elevations in urinary and blood LDH and alkaline phosphatase activity were noted, these failed to prove consistent, or to be specific indices of renal rejection, or to correlate with the level of measured renal function. LDH urinary enzymes as determined by paper electrophoresis disclosed a heretofore undescribed shift from a high, fast-moving fraction (I) in renal-allografted or renal-autografted animals. The shift is therefore not related to renal rejection, but more to nonspecific ischemic cellular destruction. Techniques of starch gel and paper urinary isoenzyme measurements are compared, and the necessity of fresh catheterized specimens for accurate enzymatic determination is stressed.