Rh34 antibodies were found to have a unique, previously only partly characterised, specificity within the Rh system. No evidence was seen that they were mixtures of hrB and Hr-like antibodies, or that hrB antibodies existed independently in the natural state. The term anti-hrB applied to Rh34 antibodies after they had been partially absorbed with R2R2 red cells. Four haplotypes not expressing Rh34 antigen were identified in the present study. The prefix * has been used to indicate them in this text. They were *r's (*dCces), *Ro (*Dce), *Rou (*D(uce)) and *R(od) (category III *Dce). Red cells with partially deleted or Rh(null) phenotypes were therefore not the sole red cells compatible with anti-Rh34. R2R2 red cells, which are known to carry weak Rh34 antigen, were incompatible. Twenty-two family and mother-child studies established that the Rh:-34 haplotypes were inherited as normal Mendelian dominant characters. Anti-Rh34 was capable of recognising RH 34 dosage and of excluding some men who had been wrongly accused in disputed paternity tests.