Chorion biopsy was performed in 165 cases at 6-12 weeks of pregnancy, following an ultrasonic or embryo-fetoscopic chorion frondosum localization. One hundred patients had their biopsies taken immediately before induced abortion. In 39 cases abortion was carried out 5-10 days after biopsy. In 26 pregnant patients biopsy was performed for genetic reasons. Fetal sex was determined in 'native' smears from biopsy specimens for cytological investigation, using X-and Y-chromatin assays. Fetal sex diagnosis proved correct in all the cases. In 40 observations, the origin of the biopsy specimen was histologically checked. In 16 biopsy specimens, a number of enzymes were simultaneously assayed : ,t?-D-ghcosidase, /3-D-galactosidase, ,t?-D-hexosaminidase, /3-D-glucuronidase, a-L-fucosidase, 8-D-mannosidase, sphingomyelinase and arylsulphatase A. The levels of the above enzymes were compared to those observed in tissue cultures of amniotic cells obtained through amniocentesis at 16-18 weeks of pregnancy. The amniotic sac remained intact in all cases of chorion biopsy. If the pregnancy was maintained after the biopsy, no spontaneous abortions were recorded, and pregnancies resulted in the timely delivery of full-term healthy infants. Therefore, the method described is a valuable means of diagnosing inherited disorders, with promising applications in prenatal medicine.