The immune response to the i.v. injection of 1 ml of D-positive (ccDEE, presumably cDE/cDE) red cells was studied in 12 D-negative (ccddee) subjects who received a simultaneous i.m. injection of 5 microgram anti-D (the test group) and in a further 12 D-negative subjects who were not given anti-D (the control group). In all cases the red cells were labelled with 51Cr. Further injections of 1 ml of red cells were given at 7 and 12 months to subjects who had not made serologically detectable anti-D. In the test group the rate of clearance of the first injection of red cells was very variable, 99% of the cells being cleared in a period ranging from 3 to 20 d. Within 6-10 weeks of the first injection four subjects had produced anti-D; four more subjects produced anti-D after the second injection of red cells. In the control group Cr red cell survival following the first injection was normal in six cases and curtailed in the remaining six. Of the latter, four produced anti-D within 4-10 weeks of the first injection and two produced anti-D only after the second or third injection of red cells. Amongst the subjects who produced anti-D after the first injection of red cells antibody levels were lower in the test group than in the control group, indicating that the injection of 5 microgram of anti-D with 1 ml red cells had not augmented the immune response and might have partially suppressed it.