An intramuscular injection of 100 mug of anti-Rh, given 13 days after an intravenous injection of 1 ml of Rh-positive red cells appeared to suppress primary Rh immunization: at 6 months, none of thirteen subjects so treated had detectable anti-Rh in their plasma, whereas anti-Rh was present in five out of twelve control subjects injected with Rh-positive cells alone (P equals 0.015, Fisher's exact test, one-tailed). Primary immunization was not suppressed in all treated subjects since, following a second injection of Rh-positive cells, 7-day survival was subnormal in three subjects, all of whom had anti-Rh in their plasma after a further 2 weeks. In three other treated subjects, primary Rh immunization appeared to be completely suppressed: survival was normal, or initially normal, following a second injection of Rh-positive cells and anti-Rh was detectable only after a third injection.