The response to 0.8 ml of D-positive (ccDEE) red cells injected i.v., together with 1 microgram anti-D i.m., was studied in 13 D-negative male volunteers (test subjects); the red cells were labelled with 51Cr and were found to have a mean survival time of 24 d, after correction for Cr elution. Within the following 5 months, nine of the 13 subjects made anti-D, compared with four out of 12 control subjects in a previous series who received 1 ml of red cells from the same donor but without passively-administered antibody. The time at which anti-D was first detected and the maximum concentration of anti-D attained were similar in test and control subjects. After a second injection of D-positive red cells, given 5 months after the first, the total number of subjects making anti-D became 11 out of 13 in the test series, compared with six out of 12 in the previous control series. Antibody concentrations were similar in test and control subjects. Evidently, Rh immunization was not augmented if this term implies accelerated formation of antibody or an obvious increase in antibody concentration. On the other hand, the suggestive increase in the proportion of responders, though not statistically significant, is consistent with the earlier observations of others and suggests that the effect of a small dose of passively administered antibody is to facilitate primary immunization.