1. Platelet counts decreased progressively in heparinized blood which had been stored in the icebox, then rotated in glass tubes. The platelets in similarly treated citrated human blood were stable.
2. Platelets and bacteria did not adhere in rotated heparinized and oxalated plasma from normal persons, or in citrated plasmas containing high concentrations of placenta or artery juice, purified fibrinogen, albumin, or gamma globulin, or in citrated plasmas from cases of subacute bacterial endocarditis.
3. Platelet-bacteria mixtures were found in rotated citrated plasmas containing saline extracts of human heart, heart valve, and vein tissues.
4. The degree of platelet-bacteria adhesion induced by sedimentation of these elements was variable. The degree of platelet-bacteria mixed clumping was enhanced by addition of purified gamma globulin and juices from human placenta, heart, heart valve, and vein, but not in plasma containing artery juice or purified fibrinogen.
5. The data indicated that bacterial agglutinins are not responsible for the adhesion of human platelets to bacteria.
6. The metabolic in vitro products of the alpha, beta, and gamma streptococci did not cause alterations in the morphology or adhesiveness of platelets.
7. Saline extracts of the human placenta, heart, heart valve, vein and artery used in these studies exhibited thromboplastic and prothrombic activity, but no thrombic activity.
8. The nature and relationship of the plasmatic factors responsible for the adhesion of platelets to bacteria, and for the agglutination of platelets, was discussed.