SummaryImmunofluorescence, tube agglutination, and platelet factor‐3 immunoinjury tests for detecting antiplatelet antibody were compared using a heterologous system of equine platelets and rabbit antiequine platelet serum. Platelet immunofluorescence tests were performed using paraformaldehyde‐fixed platelets in suspension as well as in air‐dried smears on glass slides (solid phase). Bright homogeneous, membranous, specific fluorescence was seen in both assays with anti‐immunoglobulin G (IgG) and protein G fluorescein isothiocynate conjugates (FITC‐conjugates). Protein A conjugate gave nonspecific fluorescence irrespective of normal or antiserum treatment. Anti‐IgG and protein G conjugates in suspension immunofluorescence tests with the same antiserum yielded antibody titers of 1:1024 and 1:128, respectively. Similarly, respective titers of 1:512 and 1:64 were obtained with solid phase immunoassay. Platelet suspension assay was slightly better than the solid phase assay. These observations indicated that anti‐IgG was more sensitive than protein G in detecting antiplatelet antibody by fluorescence microscopy, while protein A was ineffective because of its nonspecificity. Chloroquine treatment of platelets failed to reduce the nonspecific fluorescence. Platelet agglutination and platelet factor‐3 tests were relatively less sensitive to detect equine antipl