As a consequence of the reduced occurrence of maternal sensitization to Rh antigens, the relative clinical importance of irregular red blood cell antigen sensitization has increased. While such sensitization is reported to account for 5% of all neonatal immune hemolytic disease, most information concerning the more than 40 antigens that can cause hemolytic disease has been derived from sporadic case reports.’ Recommendations so based are subject to error and require updating when new information is obtained. Sensitization to members of the subgroups of the Kell (K) antigen (Kp”, Kpb, Js”, Jsb) has been reported to occur only occasionally and to result in mild hemolytic disease. Expectant management without serial amniocentesis has been suggested.’ This report details the occurrence of severe erythroblastosis fetalis resulting from anti-Kpb sensitization.