In 13 patients with abortion in the second to fourth months of pregnancy the variation in oestrogen levels in the plasma was determined before and after abrasion of the abortus, using the radioimmunoassay method, and correlated with the results of histological examination of the abraded parts. Seven women without endometritis revealed high oestrogen concentrations before and after the abrasion (mean (1734 pg/ml or 397 pg/ml, respectively). On the fifth day after abrasion the levels corresponded with those of the proliferation phase of the normal menstrual cycle (mean 115 pg/ml). In six women with histologically established endometritis low oestrogen levels were found before and after the abrasion, the mean values being 101 pg/ml and 36 pg/ml, respectively. On the fifth day the values continued to remain low (average 25 pg/ml). The difference in oestrogen concentrations of both groups was significant and correlated well with the results of histological examination of the abraded parts.