In his studies on the chemistry of malignant growths, Beebe (1) found that old degenerated earcinomata and sarcomata contained much more calcium than did young, rapidly growing tumors. Likewise, Clowes and Frisbie (2) observed a low calcium and high potassium content in rapidly growing mouse tumors, and the reverse in tumors of slow growth. These observations suggested the possibility of a disturbance of calcium metabolism in neoplastic disease; and, with a view to demonstrating such a disturbance, the determination of the blood calcium was undertaken in a series of cancer cases. As a basis for comparison with the values obtained in cancer, the calcium content of the blood of a number of patients presenting no evidence of malignant disease, was ascertained. The method employed was that of Halverson and Bergeim (3). The technique of these authors was closely followed, except that the protein precipitation was done in a 50 cc. centrifuge tube and centrifugalization resorted to in order to facilitate removal of the protein precipitate. Duplicates were done in all cases. The data of thirty-four patients with malignant disease are presented in table 1. The calcium content of the blood plasma varied from 5.87 to 15.05 mgm. per 100 cc, the average being 9.41 mgm. If the type and location of the tumor are disregarded, the calcium concentration of the plasma of the male is slightly lower than that of the female, the mean figure for the former being 9.42 mgm., for the latter 10.21 mgm. These average values may be considered to be within normal limits, Howland and Marriott (4) having found from 10 to 11 mgm. per 100 cc, and Halverson, Mohler, and Bergeim (5) from 9 to 11 mgm. in a series of normal individuals. In 64 per cent of the inoperable abdominal cases, the calcium content exceeded 10 mgm., but a truly characteristic calcium value for any definite type or location of neoplasm is not in evidence. In six benign tumors (table 2) the results were similar to those obtained in malignant disease; they varied from 7.65 to 12.51 mgm., and averaged 10.14 mgm.