A sensitive and simplified radioreceptor assay for 1, 25-dihydroxyvitamin D (1, 25-(OH)2D) in human plasma was described and applied to preliminary clinical studies. Tritium-labeled 1, 25-(OH)2D3 was produced by incubating chick kidney homogenate with tritium labeled 25-hydroxyvitamin D3 (25-OHD3). A cytosol receptor was obtained from rachitic chick intestine (Kd=5.3 X 10(-11) M). Lipids in 5 ml of heparinized human plasma were extracted with dichloromethane, and 1, 25-(OH)2D was isolated by a Sephadex LH-20 column followed by high pressure liquid column chromatography. Recovery of 1, 25-(OH)2D3 after the plasma extraction and chromatography ranged from 58 to 100%. The assay was sensitive to 5 pg/tube. Diluted plasma from a patient on a high dose of 1 alpha-OHD3 showed a dilution curve parallel to the standard curve. The cytosol receptor showed a cross reactivity to various vitamin D3 metabolites physiologically present in the circulation and it was thought to be essential to eliminate other vitamin D3 metabolites 1,25-(OH)2D from plasma samples by high pressure liquid chromatography. Plasma concentrations of 1, 25-(OH)2D were, in the case of most normal subjects, distributed from 7 to 33 pg/ml and the range of distribution became greater in relation to age, indicating that plasma values should be matched to age. Whereas markedly high values of 1, 25-(OH)2D in plasma were found in some cases of primary hyperparathyroidism with prominent bone resorption, relatively low values were seen in some patients with chronic renal failure, senile osteoporosis, osteomalacia and hypercalcemia due to bone metastasis.