Fetal cells entering the maternal circulation during pregnancy constitute a potential source for safe and reliable non invasive prenatal diagnosis. However, selecting the appropriate fetal cell type and methods of enrichment are areas of paramount importance. Most investigators consider fetal nucleated red blood cells (NRBCs) to be the cell type of choice, since they are mononuclear, abundant in fetal blood, relatively well differentiated and have a limited life span. Twenty ml of peripheral blood samples were collected from 40 pregnant women in the 16th to 18th week of pregnancy. To enrich for NRBCs, found within an excess of maternal cells, negative magnetic cell sorting (MACS) was used. Leukocytes were depleted from maternal blood by treatment with anti CD45 monoclonal antibody, as this surface antigen is not expressed in NRBCs. NRBCs were detected in 35 of the 40 maternal samples with May Grunwald-Giemsa staining. In 30 cases UCH gamma positive cells were identified after immunophenotyping with a monoclonal antibody directed against the gamma chain of fetal hemoglobin. The mean number of isolated NRBCs was 6 (range 1-15). In 5 cases we were able to successfully perform FISH on the immunophenotyped cells and determine correctly the fetal gender using X and Y chromosome specific probes.