17 patients suffering from visceral malignant disease were divided into two groups, one of 8 patients with hypercalcaemia secondary to neoplastic osteolysis, the other of 9 patients in whom this was not present. The patients underwent an intravenous infusion of 25 μg/kg mithramycin at a constant rate over 3 h. A study was made of changes in phosphorus and calcium metabolism and of renal function. (1) In 8 hypercalcaemic subjects, serum calcium fell from 133.6 ± 14 to 120.0 ± 18.4 mg/l (p < 0.01). There was a fall in calciuria and in the UVCa/UVinulin ratio. Serum phosphorus and serum magnesium showed no change. Serum potassium showed a significant reduction (p < 0.01) during this study. Renal function, as shown by creatinine, inulin and PAH clearances, was not reduced. (2) In 9 normocalcaemic subjects, the fall in serum calcium was less pronounced, from 92.4 ± 2.4 to 86.6 ± 2.9 mg/l. There was an increase in phosphaturia during the last hour of the test. There was also a fall in serum potassium. Renal function remained normal. The action of mithramycin is discussed. It appears to prevent bony resorption and to act on osteoclasts. Its effects appears to be limited by a secondary secretion of parathormone. The effect of mithramycin on calcaemia varies in degree from individual to individual, is transient, and is often insufficient to restore serum calcium levels to normal.