A bleeding disorder due to abnormal platelet function occurs in Simmental cattle. Whole blood from these animals underwent good clot retraction. Platelet aggregation in response to adenosine diphosphate (ADP) and collagen in a whole blood aggregation system was markedly impaired. Normal bovine platelets in a whole blood aggregation system showed very little aggregation in response to epinephrine and arachidonic acid. Aggregation in platelet-rich plasma was negligible in response to ADP, collagen and thrombin. Dense granule release of radiolabelled serotonin from the platelets of one affected cow was similar to that of normal bovine platelets. Platelet membrane glycoprotein electrophoresis with the platelets of one affected cow revealed no quantitative abnormalities. These findings reveal similarities and differences in thrombopathic Simmental platelet function when compared to human Glanzmann's thrombasthenia and Basset Hound thrombopathia.