Neonatal alloimmune thrombocytopenia (NAIT) is usually induced by platelet- specific antibodies against HPA-1a (Zw^a) or HPA-5b (Br^a). Recently, low-frequency alloantigens on the platelet glycoprotein (GP) IIb/IIIa complex have been discovered as a cause for NAIT. In this report, a new low-frequency platelet- pecific alloantigen, ly, is described which induced severe NAIT. The corresponding antigen was detected in 1/249 unrelated German blood donors. Antibody binding assays with trypsin-digested platelets (ELISA, immunoprecipitation with biotin-labelled platelets) indicate that the antigen is not localized on the glycocalicin moiety of GP Ibα, but may be situated on the remnant moiety of GP Ibα, GP IX or GPIbβ. Apparently, ly is not related to the HPA-2 (Ko) antigen system.