The binding constants of the alpha(1)-foetoprotein of the rat embryo serum for oestrone and oestradiol-17beta have similar values, i.e. 1 x 10(8) M(-1) in average at 25 degrees. There is probably one binding site per mole of binding protein. The high alpha(1)-foetoprotein concentration in the rat embryo serum at 17-19 days of pregnancy explains the exceptionally high levels of fixation of the phenolsteroids by this serum.