A dispermic human chimaera is described who is a fertile female XX/XY chimaera. The chimaera was discovered due to anomalies found during routine antenatal testing. The patient had two red cell populations differing in three blood group systems; ABO, Rh and MN. Analysis of cultured lymphocytes showed 70% of cells to have the 46, XY male karyotype and analysis of cultured skin cells showed 30% of cells to have the 46, XY karyotype.