Using acrylamide gel electrophoresis the serum alkaline phosphatase (ALP) isoenzyme patterns of 204 patients with chronic renal failure have been examined for periods up to 18 months in length. 2. Of those with elevated serum ALP activity the bone isoenzyme was largely responsible. The presence of increasing amounts of the bone isoenzyme even if the total serum ALP activity remains within the normal reference range should also indicate bone pathology. 3. Intestinal alkaline phosphatase was the major serum alkaline phosphatase in 15% of patients on regular haemodialysis and 10% of uraemic patients not on dialysis. The overall incidence of detectable intestinal alkaline phosphatase in those with normal serum ALP activity was 36%. 4. With those patients whose serum ALP activity changed significantly during the investigation this usually reflected changes in the amount of the bone isoenzyme. Patients with abnormal amounts of the intestinal isoenzymes in their serum usually showed little variation in serum ALP activity over the period of the study.
