The platelets of 11 patients with Glanzmann's thrombasthenia and their nearest family members were studied for the expression of the platelet-specific alloantigens of the Zw-, Ko- and Bak systems. The strength of the expression of the Zwa antigen was diminished on the platelets of 3 patients, and the antigen was absent from the platelets of the other 8. The platelets of none of the patients reacted with anti-Zwb serum. Therefore, Glanzmann's thrombasthenia is probably a "Zw-null disease." The expression of the Zwa antigen on the platelets of all the relatives was normal, as indicated on the cytoflurograph. Investigations on the expression of the Koa antigen were complicated by agglutinations of the platelets from genetically Koa-negative thrombasthenic patients with the anti-Koa serum. The Kob antigen was normally expressed. The Baka antigen was absent from the platelets of all thrombasthenic patients and a relatively high percentage of the relatives. A close association between the Glanzmann gene and the Bak(a-) gene is assumed on statistical grounds. Thrombasthenic platelets showed no reaction with EDTA-dependent antibodies, which are reactive with all normal platelets. Owing to immunization by multiple blood and platelet transfusions, serum samples of most patients studied contained HLA antibodies and platelet-specific alloantibodies. However, antibodies directed against the Zw-antigen-bearing glycoproteins were detected in the serum of only one patient and, therefore, seem to be rare.