Résumé : The conversion rate of estrone to 17β-estradiol was investigated in a 3-day culture of human blood mononuclear cells. Per 10 million cells the mean percentage of conversion was 5.64 (S.E., 0.24) in 56 normal subjects, and there was no significant difference among men, premenopausal women, and postmenopausal women. The conversion was not modified by the addition of steroid sex hormones in the medium or by an increase of the glucose concentrations. It was significantly lower (p<0.001) in 22 patients with a localized mammary carcinoma (mean, 3.77; S.E., 0.26) and in 16 patients with localized nonmammary carcinomas (mean, 4.28; S.E., 0.39) but returned progressively to normal or high values when the tumors became generalized. In the patients with mammary carcinomas, it was significantly lower (p<0.025) than in those with nonmammary carcinomas but was still, as in normal subjects, definitely higher than the reverse (17β-estradiol to estrone) reaction. The low estrone to 17β-estradiol conversion rate appeared to be present at the very beginning of the disease; its increase with the generalization of the tumor might be attributable to an inactivation of the monocytes because a positive correlation (r=0.554; p~0.02) between the number of monocytes and the conversion rate was then detected; this increase might also be a useful and early index of the spread of the cancer.