A healthy infant was born at term by elective cesarean section to a 32-year-old para 4, gravida 4, mother. Within 24 hours, the infant was noted to have fairly extensive bruising on the back and shoulders. A full blood count evaluation was remarkable for severe thrombocytopenia (platelet count of 29 × 109/L). Other hematologic parameters were normal. Human leukocyte antigen (HLA) class-1 antibodies but not platelet-specific antibodies were detectable in the maternal serum using a commerical antigencapture ELISA (GTI-PakPlus kit®). Anti-HPA-3a antibodies, while weakly reactive in the monoclonal antibody immobilization of platelet antigens (MAIPA) assay in the immediate postpartum serum, were readily detectable using this assay in a sample taken 4 weeks later. Genotyping for human platelet antigens (HPA) 1–5 by the polymerase chain reaction technique with sequence-specific primers (PCR-SSP) revealed the infant’s platelet genotype to be