For the purpose of elucidating the relationship between the spreading-out test proposed by Breddin and the current routine tests of platelet functions, the spreading-out index was studied in comparison with adhesiveness, aggregation, factor-3 and clot retraction. The results obtained are as follows. Positive correlation was observed between the spreading-out index and platelet adhesiveness. Non-adhesive platelets obtained by filtering the platelet rich plasma through a glass wool filter was found to have a lower spreading-out index than the untreated platelets. Therefore, it was considered that the adhesive platelets played a role in the spreading-out phenomenon. Although the adding of adenosine, AMP and ATP suppressed the spreading-out of the platelet, the adding test of ADP induced a tendency to increase the spreading-out index. The above result corresponded with the relationship between platelet adhesiveness and adenine nucleotides, and also supported the finding that spreading-out test is closely related to platelet adhesiveness. No significant correlation was obserbed between the spreading-out index and the other platelet functions, i.e., aggregation, platelet factor-3 and clot retraction.