Three newborn babies with congenital immune amegakaryocytic thrombocytopenia are described. Two were siblings. The mothers' sera showed antibody against both pooled random platelets and lymphocytes of type HLA-A2. Two babies had transient thrombocytopenia and leucopenia and made a full haematological recovery. One baby died aged four hours having had no haematological investigations during life. Histological examination of the bone marrow in all three babies (two by trephine biopsy; one at necropsy) confirmed a deficiency of megakaryocytes. It is suggested that anti-HLA-A2 may cause neonatal thrombocytopenia by depressing megakaryocytes instead of, or in addition to, any direct effect on platelets.