Sixty-nine out of a kindred of 100 members covering five generations were examined and studied, and 34 were hypercalcemic. Sixteen subjects were believed, on the basis of laboratory and clinical observations, to have primary hyperparathyroidism. Eight patients were subjected to exhaustive study to identify polyendocrine involvement before neck exploration. No coexisting endocrine abnormalities were found; operation showed multiparathyroid gland involvement in most instances. Measurement of immunoassayable calcitonin and assessment of renal and gut function were carried out in 37 subjects to search out possible causes of "reactive" parathyroid gland hyperfunction. While no such cause-effect relationship was noted for this kindred, 16 of the subjects so tested had serum calcitonin content below the assay limits of sensitivity. What role this apparent lack of calcitonin played in the development of hyperparathyroidism (or vis versa) needs clarification.