After 14 days of 1 mug ethinyloestradiol per kg body weight per day, both sexes show highly significant increases in serum coeruloplasmin which are independent of the determination method. Expressed on a percentage basis, the concentrations of coeruloplasmin under oestrogen medication determined by the p-phenylendiamine oxidase reaction are significantly higher than those determined by the immunological-nephelometric method. This is because several coeruloplasmin components, mainly C and DO, take part in the determination with p-phenylendiamine oxidase, whereas the immunological method measures chiefly the coeruloplasmin DO component. The significant sex differences under oestrogen medication shown by the determination with p-phenylendiamine oxidase, can therefore be explained by the presence of C- and D-coeruloplasmins in women, while the serum of men contains almost exclusively D-coeruloplasmins. Under oestrogen medication, the increase occurs primarily in the DO- coeruloplasmin component, and women also show an increase in coeruloplasmin-C. The significant higher enzymatic concentrations in the serum of the women in contrast to the men are thus plausibly explained. With the immunological assay, the sex differences were only probably significant.