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).