Platelet immunofluorescence, together with other serologic tests on platelets, lymphocytes, and granulocytes, was used to investigate the sera of 38 mothers with newborns who suffered from thrombocytopenia. In sera of 33 mothers, platelet-specific IgG alloantibodies were demonstrable. Three sera also contained HLA antibodies, of which two were only detectable in the lymphocyte cytotoxicity test. Two other sera contained granulocyte-specific alloantibodies. In sera of 2 mothers, antibodies were found that reacted with all cell types in all tests. However, after further analysis, it became clear that platelet-specific alloantibodies were probably also present in these 2 sera. In 29 cases, the specificity of the platelet alloantibodies was anti-Zwa--PlA1. One serum contained antibodies directed against a new antigen, Baka. This new antigen was defined after the investigation of the family and a small-scale population study. Two other sera had platelet antibodies with still undefined specificities. In all positive sera, IgG platelet alloantibodies were detected, and sometimes IgM antibodies were also present. The IgG antibodies were mostly of the IgG1 subclasses, but sometimes IgG3 and/or IgG4 was also found. In a few sera, only IgG3 antibodies were detected. In our series, we found no increased frequency of blood group ABO compatibility between mother and child, although it has been described by others and is well known to occur in rhesus alloimmunization. Of all the tests used, the platelet immunofluorescent test, especially the test on paraformaldehyde-fixed platelets in suspension, gave the best results in the detection of platelet antibodies in neonatal alloimmune thrombocytopenia.