A decrease in intestinal calcium absorption, in spite of normal serum calcitriol levels, has been reported in postmenopausal osteoporotic women, raising the possibility of an intestinal resistance to the hormone. The mechanism responsible for it could lie at the receptor or postreceptor level. Intestinal receptors are difficult to study on clinical settings, but calcitriol receptors have been found in peripheral blood mononuclear cells (PBMC). We have studied the PBMC calcitriol receptors by means of Scatchard analysis in 11 postmenopausal osteoporotic women without any treatment and in 12 normal postmenopausal women of similar age. No differences were found in the dissociation constant (Kd) or the concentration of binding sites (Nmax) (Kd in patients: 0.90 +/- 0.75 x 10(-10) M; Kd in controls: 0.85 +/- 0.40 x 10(-10) M; Nmax in patients: 2.4 +/- 1.2 fmol/10(7) cells; Nmax in controls: 2.1 +/- 0.6 fmol/10(7) cells), supporting the contention that the disorder responsible for the resistance to calcitriol in postmenopausal osteoporotic women is located at the postreceptor level. In addition, our study included five postmenopausal osteoporotic women treated with calcitriol (0.5 microgram/day). The number of calcitriol receptors was increased in this group (Nmax: 3.9 +/- 2.0 fmol/10(7) cells vs. 2.1 +/- 0.6 fmol/10(7) cells; P = 0.02).