A tetrazolium dye reduction test has been evaluated for use in assessing the suitability of cadaver kidneys for transplantation. The time required for a color change of a small cortical slice was measured. Nine human cadaveric kidney transplants were tested and the results generally paralleled the severity of the acute tubular necrosis present. In a simulated cadaver experiment with canine kidneys subjected to varying periods of ischemia, the tetrazolium test could identify kidneys which would not recover function. Its greatest potential value appears to be in the utilization of kidneys previously thought unsuitable. A favorable test result in a donor kidney which was subjected to 48 hours of severe shock and 36 hours of total anuria which was unresponsive to mannitol, resulted in a successful transplantation to a recipient who otherwise might have been denied a kidney.