A child, studied between the ages of 1½ and 3½ years, presented an abnormally low plasma alkaline phosphatase activity (0.8-1.64 Bessey-Lowry u.), a deformed skeleton and the loss of most of her deciduous teeth. The serum Ca was normal; the serum inorganic phosphate remained at the normal relatively high levels of infancy as the child grew older Roentgenograms demonstrated deficient mineralization of the skeleton and teeth. Biopsies of the liver and the costochondral junction displayed a deficiency of tissue alkaline phosphatase activity. The architecture of the rib was consistent with rickets. There was no evidence for the presence of an inhibitor of alkaline phosphatase, such as beryllium, or for an excessive excretion of the enzyme. Treatment with purified growth hormone, ascorbic acid and thiamin chloride had no effect, while vitamin D 500 thousand u. caused little change in the enzyme activity in a 10 day period. The father had low plasma alkaline phosphatase activity and a number of similar patients are mentioned, for whom there was also evidence that the deficiency in alkaline phosphatase activity may be genetically determined. While the precise role of alkaline phosphatase activity in the metabolism of bone is not clear, the findings in this patient suggest that growing bone may require the presence of alkaline phosphatase for normal calcification, and that the skeletal disorder, which could not be distinguished from rickets, may be related to a disturbance in the local factor.