Any future programme of antenatal screening of pregnancies for risk of neonatal alloimmune thrombocytopenia will have as a major requirement the availability of cost-effective assays which can be applied to large numbers of samples. To address this, we developed a competitive ELISA to type whole blood samples for the platelet alloantigen HPA1a, based on the use of purified glycoprotein (GP) IIb/IIIa from donors of known HPA1 genotype along with well characterized anti-HPA1a antiserum. Microtitre plates were coated with purified GPIIb/IIIa from donors of genotype HPA1a1a/3a3a. Anticoagulated whole blood of unknown HPA1 type was added to each well followed by anti-HPA1a. Residual antiHPA1a antibody not bound to the platelets in the test blood sample, bound to the immobilized HPA1a on the plate and was quantitated by standard ELISA. 475 blood donors were typed by the assay and the results compared in a blinded comparison with typing in the Capture-Ptm assay. Concordance was 100% (468 HPA1a positive and seven HPA1a negative). The HPA1 type of control samples stored as whole blood could be discriminated by this assay for up to 23 d of storage at 4 degrees C. This assay should be suitable for use in large-scale population screening programmes.