Antibodies to antigens in the Kell blood group system are usually immunoglobulin G, and, notoriously, anti-K, anti-k, and anti-Kp a can cause severe hemolytic transfusion reactions, as well as severe hemolytic disease of the fetus and newborn (HDFN). It has been shown that the titer of anti-K does not correlate with the severity of HDFN because, in addition to immune destruction of red blood cells (RBCs), anti-K causes suppression of erythropoiesis in the fetus, which can result in severe anemia. We report a case involving anti-Kp a in which one twin was anemic and the other was not. Standard hemagglutination and polymerase chain reaction (PCR)-based tests were used. At delivery, anti-Kp a was identified in serum from the mother and twin A, and in the eluate prepared from the baby's RBCs. PCR-based assays showed twin A (boy) was KEL*841T/C (KEL*03/KEL*04), which is predicted to encode Kp(a+b+). Twin B (girl) was KEL*841C/C (KEL*04/ KEL*04), which is predicted to encode Kp(a-b+). We describe the first reported case of probable suppression of erythropoiesis attributable to anti-Kp a . One twin born to a woman whose serum contained anti-Kp a experienced HDFN while the other did not. Based on DNA analysis, the predicted blood type of the affected twin was Kp(a+b+) and that of the unaffected twin was Kp(a-b+). The laboratory findings and clinical course of the affected twin were consistent with suppression of erythropoiesis in addition to immune RBC destruction.