It is well known that 1α,25-dihydroxyvitamin D(lα,25 (OH)2D) plays an important role in the pathophysiology of bone and calcium metabolism. In present paper, we established a relatively convenient assay system, using radioreceptor assay, for lα,25 (OH) 2D. Fundamental studies showed that this assay system had good sensitivity enough to detect 2 pg of lα,25 (OH) 2D per tube, and had good reproducibility. Therefore, it was shown that this assay system could be applied for clinical use. The plasma concentrations of lα,25(OH)2D, measured by this assay system, in healthy young males and females, and in aged females were 55.7±16.1, 44.8±22.1 and 23.5±13.0 pg/ml (mean±s.d.), respectively. Thus, the aged females showed significantly lower (p<0.005) lα,25 (OH)2D levels than the young group. This fact suggests that vitamin D deficient state might exist in aged females
