Quantitative haemagglutination studies on samples from 113 members of the tribe revealed 5 individuals whose red cells showed weak expression of Rh antigens. Two were of group R(1) R(2), 2 of group R(1) R(z), and one of group R(2)R(0). All were close tribal relatives of the propositus. It is proposed that these individuals are heterozygous for the allele for which the propositus is homozygous, and which is responsible for her Rh(null) phenotype. The case appears to be similar to that studied by Levine and co-workers, and it seems that the propositus does not express Rh antigens due to a genetically-determined lack of a precursor, which is necessary for Rh (and UW) antigen biosynthesis. The locus at which the block has occurred is not closely linked with the Rh locus.