Intensive plasma exchange was performed with a continuous-flow centrifuge in 2 pregnant women severely immunized to the Rh(D) iso-antigen. Plasma exchange was started at 17 and 18 weeks of gestation and a total of 220 litres and 185 litres plasma was replaced with plasma protein fraction over a period of 14 and 12 weeks respectively. There was a marked lowering of the anti-D antibody concentration in one patient, and the expected increase in antibody level during pregnancy was prevented in the other. Serum IgG and IgA levels were decreased by the exchange, but the continuous hypogammaglobulinaemia had no adverse effects. The serum IgM concentration was also lowered, but plasma levels returned to normal between plasma exchange procedures. The successful outcome in both cases suggests that intensive plasma exchange may reduce fetal haemolysis and that it may be an useful adjunct to amniocentesis, intra-uterine transfusions and premature delivery in the prenatal management of haemolytic disease in newborn infants.