Fetal blood sampling under ultrasound guidance has added a new dimension to the management of Rh hemolytic disease. Combined with the established parameters of history, antibody measurement and amniotic fluid ΔOD(450) readings, direct testing of the blood of the fetus completes the picture. As well as giving information that may be applied to decisions regarding management, an opportunity is provided to observe the mechanism of anti-D IgG-induced red cell destruction in vivo.