The blood of 100 pregnant women was screened to detect Y-chromatin containing cells of the fetus for prenatal sex-determination. Obtaining 15% false predictions the previous rate of wrong diagnoses could not be reduced significantly. Studying the frequency of fetal leukocytes in the maternal circulation, a significant increase during the pregnancy could not be found. In early pregnancy lymphoid cells containing Y-chromatin can be found first in the 7th week and granulocytes in the 10th week. The prenatal fetal sex-determination from 13 pregnant women was done by chromosome studies, too. Their rate of false diagnoses was 0%. Screening the Y-chromatin containing cells of 13 women, who had given birth to a male child within the last 5 years, we were able to establish a survival of fetal lymphocytes in the maternal circulation within 25 months. An increase in fetal cells according to the length of time the cells were cultured, could not be detected.