BACKGROUND Very few people have no Kell antigens (phenotype Ko). They can develop antibodies to Kell antigens after transfusion, or the abortion of a Kell-positive fetus. This paper describes a case of immunization that may have been due to amniocentesis. METHODS The eighth pregnancy of a woman required an amniocentesis on the 17th week for chromosomal study because she was 41 years old. She had 4 prior abortions. Her blood group was A Rh+. Her red cells were not tested for rare blood groups and antibodies to blood groups were not screened before and after amniocentesis. The newborn baby developed hemolytic anemia. On her 10th hour of life, her hemoglobin was 10.7 g% and her bilirubinemia 308 mumol/l. Her blood group was A Rh+. Indirect Coomb's test was positive in the mother, and the baby was given 3 exchange transfusions of O+, Ccee, K- blood. Further studies showed that the mother had phenotype Ko (A+, Ccce, K-, k-, Kpa-, Kpb-, Jsa-, Jsb-). The baby's phenotype was K-, k+, Kpa-, Kpb-, Jsa-, Jsb+. The mother was found to have a high titer of Ku antibodies. CONCLUSIONS This mother belongs to one of the 3 families known in the Reunion Island to have phenotype Ko. She had never been given transfusions, and prior abortions are unlikely to have played a role since no hemolysis was seen in further newborns. While amniocentesis is probably a major factor, its role cannot be determined because no pre-amniocentesis samples were analysed immunologically.