The results of the study confirm the results of previous studies concerning detection of three-signal nuclei from trisomy 21 pregnancies after enrichment for fetal cells in maternal blood by FACS (Bianchi et al., 1992; Simpson et al., 1994) or MACS (Ganshirt-Ahlert et al., 1992, 1993; Al-Mufti et al., 1999). The application of FISH with LSI 21 specific probe in maternal blood enriched for NRBCs can be potentially used to select pregnancies at high risk of trisomy 21 for invasive testing.