We have studied the arrangement of Rh (rhesus) genes in donors who are completely null for the products of one of them, RHCE. We show that five of six homozygous individuals with the so-called Rh D-- phenotype, who express no red-cell antigens of the C/c and E/e series, have rearranged RHCE genes in which internal sequences have been replaced by the corresponding sequences from RHD. Moreover, although there is heterogeneity at the 3' end, the 5' boundary of this chimerism is within the same small interval around exon 2. This interval is characterized by an exceptionally high degree of sequence homology between RHCE and RHD, a high density of dispersed repetitive elements, and the presence of an alternating purine-pyrimidine copolymer tract. We suggest that these features may explain the mechanistic basis for the origin of the rearrangement.