Abstract: A method is presented for selective stimulation of fetal lymphocytes to divide in culture after treatment of mixed adult-fetal blood samples. This method is particularly useful for diagnosing genetic abnormalities of a fetus using fetal cells from a maternal blood sample. The method consists of preferential destruction of maternal lymphocyte response to low concentrations of mitotic stimulants which results in a relative increase in the number of responding fetal cells in mixed adult-fetal blood samples. In the preferred embodiment of the invention, blood samples from pregnant women are processed by the described methodology, in particular, the erythrocytes are removed from the maternal sample by centrifugation, filtration or preferential lysis with ammonium bicarbonate, the suspension is then subjected to an osmotic shock and exposed to mitotic stimulants in culture, and cells in mitosis are harvested for chromosome analysis.