Anti-PP1Pk is produced by p individuals without prior red blood cell alloimmunization. This antibody can react over a wide thermal amplitude, has the potential to bind complement, and has caused hemolytic transfusion reaction, hemolytic disease of the fetus and newborn, and a high rate of spontaneous abortions. This report of two cases describes the genetic basis of p phenotype underlying anti-PP1Pk production and the development of a semi-nested polymerase chain reaction (PCR) assay for screening this observed mutation among Thai blood donors. Antibody detection and confirmation were examined by serologic testing. Genomic DNA was extracted from two Thai Burmese descendants with the p phenotype and a history of spontaneous abortions caused by anti-PP1Pk; the entire coding region of the