Bone diseases may in most instances be recognized by x-ray and clinical examinations. In certain cases, however, correlation of all available data, including laboratory criteria, is necessary for a correct diagnosis. An attempt will be made here to review calcium, phosphorus, and phosphatase determinations and other procedures as they relate to the diagnosis of diseases of the bone. The serum calcium, phosphorus, and phosphatase values vary with age (11). The normal serum calcium ranges from 9.5 to 10.5 mg. per 100 c.c. for adults, 10.0 to 11.5 mg. for children, and 10.5 to 12.0 for infants. The serum phosphorus ranges from 2.5 to 4.0 mg. per 100 c.c. for adults, 4.5 to 5.5 mg. per 100 c.c. for children, and 5.5 to 6.5 mg. per 100 c.c. for infants; it should be determined on serum which is free of hemolysis. The serum calcium is composed of two fractions: the diffusible calcium (nearly all ionized) and the non-diffusible calcium; the latter constitutes about 45 per cent of the total and is bound to protein. Hence, its amount depends on the amount of protein or, somewhat more specifically, on the amount of albumin. That protein and calcium are closely bound has been demonstrated by ultracentrifuge experiments. The effect of the protein concentration on total calcium has been emphasized by McLean and Hastings (13) and by Albright and his co-workers (2). McLean and Hastings devised a graph for calculating the ionized fraction and the protein effect. When there is no renal insufficiency or hyperglobulinemia, 1 gram of protein binds approximately 0.75 mg. of calcium. (This is only an approximation but gives an idea of the effect of the lowering of serum protein by 1, 2, or 3 grams, as in nephrosis or malnutrition.) Failure to recognize the protein effect sometimes obscures an actual hypercalcemia in hyperparathyroidism, or in other instances may lead to the false assumption of hypocalcemia. The phosphatase activity of serum has been extensively studied for many diseases. On the basis of activity at different pH ranges, phosphatases may be divided into four types: (1) an alkaline type with optimal activity at about pH 9.3, found in bone, ossifying cartilage, intestine, kidney, mammary gland, lung, spleen, blood serum, leukocytes, and the adrenal cortex; (2) a phosphatase with optimum activity at pH 6.0, found in mammalian erythrocytes; (3) a phosphatase with optimum activity at pH 5.0, found in spleen, liver, pancreas, kidney, prostate, and serum; (4) a phosphatase in certain yeasts with optimal activity at pH 3.0 to 4.0. Clinically we are concerned with two of these groups, namely, the alkaline at pH 8.6 to 9.3 and the acid at pH 5.0. The erythrocyte phosphatase can be dismissed with the statement that hemolysis-free serum should be used for the determination of both phosphatases, but especially the acid phosphatase. Unfortunately, many methods for determining phosphatase activity are in use, each with a different set of standards.