Vitamin D-deficiency and rickets was produced in growing chicks. The resulting decrese in mineralization of whole bone and of fractions separated by density centrifugation was accompanied by a very significant decrease in the contents of O-phosphoserine and O-phosphothreonine. Likewise, the total amount of O-phosphoserine and O-phosphothreonine and the concentrations of these phosphoamino acids in EDTA extracts and in fractions obtained by molecular sieving was also reduced. These data provide the first in vivo evidence that phosphoproteins may be critically involved in the calcification of bone.
