Abstract As tested by anti‐A and anti‐B sera, blood drawn from 30,000 inhabitans of Bogotá, Colombia, showed the following phenotype percentages: A — 26.5, B — 8.0, AB — 1.1, and O — 64.4. The allele frequencies were: A — 0.150, B — 0.048, and O — 0.802. In the same samples anti‐D (Rh 0 ) was used: 94.6% of the individuals carried the D antigen. All bloods were typed with fresh commercial sera. No back‐typing was performed. Hemolytic disease of the newborn, caused by Rh‐Hr isoimmunization, is more frequent (0.40%) in Bogotá than should be expected for a population having only (5.4%) or less “Rh negative” individuals. This paradoxical fact was observed in the analysis of 12,672 deliveries in an obstetrical hospital of Bogotá. A possible explanation could be the multiparity of Colombian women since the possibility of isoimmunization and hemolytic disease increases with each succeeding pregnancy in a given woman. Also it is well established that when Rh‐Hr incompatibility is combined with ABO incompatibility, it is less apt to produce disease than when it occurs alone. AO isoimmunization should therefore be less frequent in Bogotá than in populations that have a higher frequency of A individuals. Thus, the women of Bogotá are less “protected” from Rh‐Hr immunization by ABO incompatibility.