This work was presented in part at the 4th International Congress of Endocrinology, Washington, D. C., 18-24 June 1972. Dr. Palmieri is a Clinical Investigator at the Veterans Administration Hospital, Oklahoma City, Okla., and Dr. Omenn is the recipient of Career Development Award 43122 from the National Institutes of Health. Address reprint requests to Dr. Palmieri, Section of Endocrinology, Department of Medicine, University of Tennessee, Memphis, Tenn. 38163. Received for publication 26 July 1973 and in revised form 16 November 1973. from those obtained with extracts of normal parathyroid glands and parathyroid adenomas. Although both nonparathyroid neoplasms and parathyroid extracts demonstrated immunoheterogeneity by gel filtration, greater heterogeneity was found in nonparathyroid malignant tumors. In those tumors in which immunological cross-reactivity to PTH was detected, the capability of secreting PTH may be restricted to derepressed cell clones amidst other neoplastic cells, whereas the greater heterogeneity of ectopic PTH may reflect hormone cleavage by proteolytic enzymes in the tumor that is less specific than the Pro-PTH cleaving enzyme in the parathyroids.