A procedure that is simple and convenient for general use for the estimation of calcium, magnesium, and phosphorus in diet and stool is described. It includes a HNO3—HClO4 digestion in a micro-Kjeldahl flask. Calcium subsequently may be estimated by atomic absorption spectrophotometry or by the Clark-Collip procedure, magnesium by atomic absorption spectrophotometry, and phosphorus by the method of Fiske and SubbaRow. Chromium, used as a focal collection marker, does not interfere in the measurement of calcium and magnesium, whereas the measurement of phosphorus requires a preliminary ketone extraction of the chromium. The precision and accuracy of the procedure have been evaluated and found to be quite satisfactory.