Immunization of a mother negative for the platelet-specific antigen P1A1 against the P1A1 antigen of her child induced neonatal alloimmune thrombocytopenia. This prompt immune response occurred in her first pregnancy and we were therefore interested to study whether there is an association with genes located within the major histocompatibility complex. We deduced haplotypes of HLA-A, B, C and DR as well as of complement C2, C4 and Bf in the maternal family members.