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Summary. When Rh negative male volunteers were given successive small injections of Rh positive blood, it was repeatedly found that rapid clearance of the cells took place in the absence of detectable antibody. It is thought likely that primary immunisation to the D (Rho) antigen had occurred in these cases and that a similar state occurs as a result of Rh positive pregnancies in Rh negative mothers. These findings have a relevance to the assessment of trials in which gammaglobulin anti‐D is given to prevent Rh immunisation.