Chromosomal aneuploidy is a major cause of fetal loss and genetic disease. We have devised a polymerase chain reaction (PCR)-based test that allows prenatal detection of trisomy 21 in as few as 15 fetal cells within 1 day. A pair of fluorescein-tagged primers directs amplification of a 216-bp fragment of the human S100B gene on chromosome 21. Primers that direct amplification of a 165-bp fragment of the IGF1 gene on chromosome 12 are included to generate an internal standard for quantitation. After 31 cycles of PCR, the amounts of S100B and IGF1 amplification products are determined on an Automated Laser Fluorescent DNA Sequencer. In trisomic cells, the relative amount of the S100B product is approximately 1.5-fold higher than that from normal cells. The test may be useful for non-invasive prenatal diagnosis performed on fetal cells isolated from maternal blood.