A new microtechnique, C-FDA, for the in vitro detection of antiplatelet antibodies, is described. This technique is faster and simpler than either 51Cr thrombocytotoxicity or immunofluorescence (IF). C-FDA is more sensitive than 51Cr for all (anti-HLA, --P1A1, ABO, drug-related, and ITP-related) antibodies tested. Although IF was more sensitive for many types of antibodies, C-FDA was as good or better a clinical test method for all drug-related and isoimmune neonatal thrombocytopenia patient sera tested. Preliminary data also suggest that this method detects possible new non-HLA, non-ABO, nonP1A1 platelet antigens.