Liver, intestinal, and bone alkaline phosphatase isoenzymes were measured using heat stability and L-phenylalanine inhibition techniques in 78 patients on intermittent haemodialysis. Fifty-five patients had abnormalities in one or more of the isoenzymes. Changes in bone and intestinal alkaline phosphatase activities seemed to be related and raised liver isoenzyme activity was associated with the development of liver disease. Abnormal histological and radiological findings were better correlated with bone alkaline phosphatase levels than with total alkaline phosphatase, and serial estimations of bone isoenzyme activity were useful in assessing the response of renal osteodystrophy to treatment with a vitamin D analogue. Serum alkaline phosphatase isoenzyme measurement provides another useful and non-invasive index for monitoring metabolic bone disease in patients with chronic renal failure.