1. Immunochemical cross-reactivity, but not identity, has been demonstrated between bovine parathyroid hormone and an antigen in biologically active extracts of human parathyroid tissue by quantitative C' fixation and C' fixation inhibition.
2. An antigen that fixes C' with rabbit antibody to bovine parathyroid hormone has been found in urea extracts of six human non-parathyroid neoplasms associated with a hypercalcemic syndrome mimicking primary hyperparathyroidism. Comparable extracts of control tissues and other tumors were serologically negative.
3. It is concluded that the tumor antigen is parathyroid hormone or a very closely related protein, and that its production by these neoplasms was the cause of the hypercalcemic syndrome in these six patients.