RECENT reports by Hardisty, Dormandy and Hutton1, Hardisty and Hutton2, and Spaet and Cintron3 have demonstrated that incubation of platelet-rich plasma with kaolin shortens the recalcification1,2 and Russell's viper venom (‘Stypven’)3 times of the mixture. These authors concluded that the effect resulted from availability of platelet coagulant activity (platelet factor 3 availability). Since the activity is confined to the sediment of the centrifuged mixture, is related to the platelet concentration and the anticoagulant used (citrate or ethylenediamine tetraacetic acid), it appears to be related to a surface influence of kaolin, accompanied by adhesion and aggregation of the platelets. In the recalcification system, Hardisty et al.1 detected a marked abnormality with thrombasthenic platelets, which they suggested may be correlated with the failure of these platelets to react with adenosine diphosphate (ADP). We have examined nine thrombasthenic patients by the simultaneous use of the global recalcification test of Hardisty et al.1,2 and the more specific Stypven time according to Spaet et al.3. In addition the activation of Factor XII was followed and the results indicate the existence of a further in vitro defect in thrombasthenia.