Placental alkaline phosphatase phenotypes were studied in extracts of chori onic tissue from spontaneous and induced abortions. In the series of spontaneous abortions the frequency of the Plf1 gene and of phenotypes having the F1 factor was significantly higher than in the series of induced abortions and in the birth population. The over-representation of the F1 factor was more pronounced in abortions after the 12th week of pregnancy. These results suggest that there exists a pre-natal selection against the Plf1 gene.