The in vitro distribu-tion of C-14 and H-3 labelled estrone, estradiol-17[beta] and estriol between the erythrocytes and serum of horse blood has been studied over concentrations ranging from 0.0001 to 1,000 [mu]g/ml 30.2 [plus or minus] 1.1% of estrone radioactivity, 20.7 [plus or minus] 2.7 of estradiol-17[beta] radioactivity and 37.3 [plus or minus] 3.0% of estriol radioactivity was associated with the erythrocytes at a 50% hematocrit. The distributions were independent of concentration and equilibrium was achieved in less than one hour.