Of blood donations reaching the Leeds Regional Blood Transfusion Laboratory about 1% have green plasma. Most of these come from women taking oral contraceptives, who constitute about 6% of the blood-donor population. The three conditions in which green plasma is found all have elevated amounts of the blue plasma-protein, cæruloplasmin. A reduction in yellow pigments, previously reported in rheumatoid arthritis, makes the green colour easier to notice. The extractable yellow pigments of normal plasmas are mainly hæm and carotenoids. A comparison of the amount of yellow pigments in plasma from women taking contraceptive steroids, with those not on this treatment, showed no systematic difference. The rise in cæruloplasmin in women on oral contraceptives is probably due to the oestrogen components, ethinyl oestradiol or mestranol. Both of these produce the effect.