We describe a patient who developed transient and moderately severe thrombocytopenia (platelet count nadir 35 x 10(9)/L) after the transfusion of plasma. Using the technique of direct radioimmunoprecipitation, we showed that during the thrombocytopenia episode, the patient's platelets had IgG specifically bound to the glycoprotein (GP) Ia/IIa complex. Indirect radioimmunoprecipitation using serum from the plasma donor confirmed that anti-HPA-5b (anti- Zava) was the cause of GP Ia/IIa sensitization. The relatively mild thrombocytopenia, compared with passive alloimmune thrombocytopenia caused by anti-HPA-1a (anti-P1A1), may reflect the low copy number of HPA-5 compared with HPA-1. Direct radioimmunoprecipitation permits the detection of the GPs carrying the known platelet alloantigen systems, and this study suggests that this technique can be used to diagnose passive alloimmune thrombocytopenia.