Reversibility of changes induced by sexual steroids was studied in 19 different serum protein fractions of 20 women. The following preparations were available for testing: Gravistat (ethinyl-oestradiol/norgestrel), Non-Ovlon (ethinyloestradiol/norethisterone-acetate), Ovosiston (mestranol/chlormadinone-acetate), and Deposiston (ethinyloestradiol-sulphonate/norethisterone-acetate). The tests were made towards the end of the 24th cycle on the pill and in the first cycle thereafter.--The proteins tested were found to be affected in a differentiated way and, throughout, depending on oestrogen levels. Reversal was rapid, but the phase of restitution usually was longer than the period of testing and follow-up. The long-lasting action of Deposiston was visualised also in its "reversal effect".