We have developed a sensitive PCR-based assay for the RhD gene and used it to detect circulating fetal cells from RhD-positive fetuses from peripheral blood of RhD-negative mothers. With further improvement in diagnostic accuracy, this assay may have implications in the management of RhD-sensitized pregnancies in women whose partners are heterozygous for the RhD gene. Further studies are required to determine the relationship between maternal anti-D levels and circulating fetal cell numbers.