The authors describe three patients with neonatal thrombocytopenia where they detected in the maternal serum, using the immunofluorescence and immunoenzymatic test, antibodies against thrombocytes and thus confirmed the alloimmune nature of thrombocytopenia. By molecular characterization of the immunoreactivity of the thrombocytic antibody in the mother of neonate no. 1 evidence was provided that the antibody is against glycoprotein IIIa which carried antigen P1A1. In the reference laboratory in Amsterdam the anti-P1A1 specificity of this thrombocytic antibody was confirmed. The authors obtained and tested the first type anti-P1A1 serum in Czechoslovakia which can serve to detect P1A1 antigen negative subjects.