Immunoelectrophoretic investigations were carried out with water-sault extractions from the placentae of 100 in-lamb ewes manifesting normal pregnancy, studying 20 animals during each month of pregnancy. The placentae were taken immediately after slaughter. The same investigations were performed also on a total of 40 placentae of ewes that had miscarried. Precipitation was carried out with hyperimmune sera against placental protein of the first and fifth month of pregnancy. Results showed that the placenta can synthetize proteins. In the first two months of pregnancy protein synthesis is at a lower stage, but in the 3rd, 4th and 5th month it becomes more intense. Each month's proteins are characterized by a specific immunoelectrophoretic picture. At the end of the first month as many as four fractions are established electrophoretically in the placental proteins, which, by the end of the pregnancy period rise up to ten. In the course of pregnancy, along with the organospecific proteins stage-specific ones are also synthetized. These can be demonstrated through active anaphylaxis in guinea pigs. Abortions in ewes are said to disturb the normal synthesis of proteins. The "immunoelectrophoregrams" of ewes that have miscarried in the 3rd, 4th and 5th month of pregnancy reveal the drop in the intensity of the albumin fraction and the number of arcs (from 10 to 5) in the section of the alpha, beta and gamma globulins.