To improve cell recovery of trophoblast and nucleated red blood cells from maternal blood for diagnosis of chromosomal abnormalities, we have investigated the use of a magnetic sorting system that utilizes a ferrofluid. The main advantage of this system is that the beads used are small enough to allow visualization of chromosome-specific sequences by fluorescence in situ hybridization (FISH). The ferrofluid was validated using MAb340, a trophoblast-specific antibody, and anti-CD71, used to sort for nucleated red blood cells. Antigen-positive cells could be efficiently sorted from a 1000-fold excess of antigen-negative cells and easily stained by FISH. We are currently evaluating its use on maternal blood samples.