Fetal cells isolated from maternal peripheral blood during the second trimester of pregnancy were analyzed. Blood samples were centrifuged in a Ficoll-Paque gradient, the mononuclear cell fraction was isolated and stained with fluorescent monoclonal antibodies against glycophorine A (GPA + PE), transferrin (CD71 + FITC), and Hoechst 33342. Fluorescence-activated cell sorting (FACS) was conducted on a Vantage flow cytofluorimeter (Becton Dickinson). Fluorescence in situ hybridization (FISH) with Y chromosome-specific DNA probe revealed fetal cells that exhibited Y signal in all 20 blood samples obtained from women pregnant with healthy male fetuses. The concentration of these fetal cells averaged about 1.34% and ranged from 0.1 to 4.2% in different blood samples. In six cases, blood samples were obtained from pregnant women, in which prenatal cytogenetic analysis revealed various fetal aneuploidies. Using FISH with DNA probes specific for chromosomes X, 18, and 13/21, Fetal cells with chromosomal aberrations were detected in these six maternal blood samples at a concentration from 1.5 to 5.6% (on average 3.7%). These results indicate the possibility of a new noninvasive approach, which is safe for both mother and fetus when used for isolation of fetal cells from pregnant women's blood samples and prenatal diagnosis of a broad spectrum of fetal cell chromosomal aberrations.