During the past 15-20 years our knowledge concerning the phosphorus content of the blood has been considerably advanced by the work of several investigators. The inorganic phosphorus fraction, which is only about tylQ of the total phosphorus compounds of the whole blood, appears to have claimed the attention of most of the previous workers, and, therefore, abundant data are available from the literature both for health and disease. Because of this, the inorganic phosphorus fraction of the blood has already acquired some definitely practical clinical value. The other fractions of the phosphorus of the blood have attracted the attention of fewer workers, and have been less studied with the obvious result that such scanty observations as are available, are still of only theoretical interest and no clinical value. In view of this scanty information as to the total content and the distribution of the phosphorus compounds in the blood it seemed advisable further to investigate the question and the present communication is an attempt to do this using the child chiefly as the material.