The fundamental biochemical abnormality in familial benign hypercalcaemia (FBH) (familial hypocalciuric hypercalcaemia) is unknown. It seemed possible that, since the kidneys and the parathyroid glands are insensitive to the high extracellular calcium levels, a general disorder of the regulation of the calcium pump on the plasma membrane is present. We obtained evidence suggesting that active calcium efflux by erythrocytes from patients with FBH (85.7 +/- 4.5 mumol 1(-1) min-1) is higher (P less than 0.005) than that by erythrocytes from control subjects (78.6 +/- 4.1 mumol 1(-1) min-1) or from patients with primary hyperparathyroidism (77.5 +/- 5.2 mumol 1(-1) min-1, P less than 0.05). Calcium influx into erythrocytes was normal in FBH and in primary hyperparathyroidism.