We experienced a case in which anti-D was detected in an RhD-negative patient who was transfused with RhD-negative red blood cells (RBC). We requested the Japanese Red Cross Tohoku Block Blood Center to perform a genetic analysis of the blood products used, and found a typical DEL gene (c. 1227G>A) in the donor of the most recently transfused RBC. Since factors other than transfusion were negative for the production of anti-D, it was inferred that transfusion of DEL-type was involved in the production of anti-D. Although the immunogenicity of the DEL-type is considered low, there are scattered reports that transfusion of this type is involved in the production of anti-D. Therefore, it is considered necessary to avoid transfusion of DEL-type RBCs whenever possible to prevent Hemolytic Disease of the Fetus and Newborn (HDFN) in RhD-negative pregnant women and hemolytic transfusion reactions (HTR) in anti-D carriers. Since DEL-type RBCs are also known to carry at least one C antigen, and none have a cc phenotype, the use of D and C antigen negative RBCs for patients described previously is a rational choice in terms of safe transfusion, cost, and supply-demand balance.