This report presents the results of two National Workshop exercises which were designed to evaluate interlaboratory and interassay variation in sensitivity of techniques used for the detection of platelet-reactive alloantibodies. Most workshop participants used the platelet immunofluorescence test. Sensitivity of this assay was improved when fluorescence was measured by flow cytometer rather than by microscope. The MAIPA assay was found to be highly sensitive but required considerable technical expertise, and the choice of antiglycoprotein IIb/IIIa (CD61/41) monoclonal antibody was found to have a significant effect on its ability to detect anti-HPA-1a.