(1) Simple incubation of normal platelets in saline for 10 minutes at 37 C. markedly diminishes their activity in the thromboplastin generation test. This loss of activity is due to the removel of a factor(s) from the platelet and is present in the saline extract.

(2) These "attenuated" platelets retain their effect on the recalcification time, prothrombin consumption, and clotting time of whole blood.

(3) Attenuated platelets, as well as platelets thought to be qualitatively defective, regain normal activity in the TGT after incubation in normal plasma or in plasmas from patients with qualitative platelet defects.

(4) Attenuated platelets, in contrast to qualitatively defective platelets, are restored to normal, as measured by the TGT, after treatment with saline extracts of normal platelets. Attenuated platelets, however, do not function normally after incubation with saline extracts of qualitatively defective platelets.

(5) The possible mechanisms involved are discussed, and it is concluded that the phenomenon involves the adsorption of plasma factor(s) by the platelet. The nature of the plasma factor(s) is not known.

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