A quantitative study of the pregnancy zone protein in sera of woman taking oral contraceptives.

By means of a single radial immunodiffusion the concentration of "the pregnancy zone protein" (PZ) was measured in sera from women taking oral contraceptive drugs. Woman taking only 0.3 mg. of norethisterone were found not to induce measurable amounts of PZ, whereas women taking combined contraceptive drugs showed a significant rise in concentration of PZ. After six months' treatment, 1 mg. of norethisterone and 0.1 mg. of meastranol daily were found to give an average concentration of PZ amounting to 59 mg. per 100 ml., that is, approximately half of the concentration of PZ in sera from women during the last trimester of pregnancy. Different combined contraceptive pills gave rise to different concentrations of PZ.