A male infant was born to a 20-year old, primipara healthy mother, after a 38-week umcomplicated gestation. He showed generalized purpura at 2 hours of age, but no hepatosplenomegaly. His platelet count was 1.7×104/μl and had no response to platelet transfusion from random donors, but gradually increased to 32.5×104/μl on 24 days of age. Mixed passive Haemagglutination test revealed that his platelets were destructed by maternal platelet specific antibody, passively come through placenta. After analyzing specificity of the mother's serum, it was not identical with any of the known platelet specific antisera. So, we diagnosed this case as a neonatal alloimmune thrombocytopenia due to a new platelet specific antigen (Yuka). It might be possible that other same cases will be found in Japan.