We have been unable to verify the observations of earlier writers that there is an abnormality of the calcium metabolism in arthritis deformans. We have found no abnormality in the serum content of either the calcium phosphorus, or the calcium-phosphorus product in arthritics as compared with normal adults. Furthermore, no reliable distinction is offered by these analyses for the separation of rheumatoid arthritis from osteo-arthritis, the only difference noted by us consisting of a tendency of the phosphorus readings of the osteo-arthritic group to be lower than those of the rheumatoid group. Finally, our observations would show that the activity of the disease, whether measured by the clinical symptoms or by the objective bony overgrowth, is in no way indicated by the results of these analyses.