1. Contrary to the commonly accepted view, vasodilatation does not always precede the adherence of leucocytes to vascular endothelium.

2. The fact that leucocytes often adhere to one another as well as to the endothelium indicates that the increased adhesiveness characteristic of the early stages of inflammation is not limited to the surfaces of the endothelial cells.

3. The sharing of erythrocytes and platelets in this increased stickiness suggests that a "plasma factor" is involved. There is indirect but as yet inconclusive evidence that the plasma factor may concern the clotting mechanism of the blood.

4. The adherence of leucocytes to the endothelium is usually first noted on the side of the vessel closest to the site of injury. This previously undescribed phenomenon of "unilateral sticking" is in keeping with the concept that the vascular reaction is caused by products of cellular damage which diffuse to the vessel from the site of injury.

5. Leucocytes always become adherent to the endothelium before penetrating the vessel wall. They often migrate about for some time on the endothelial surface before undergoing diapedesis.

6. Although no definite stomata are at any time visible in the endothelium, penetrating leucocytes may leave behind temporary defects through which other leucocytes and even erythrocytes may pass.

7. The diapedesis of leucocytes appears to depend primarily upon cellular motility. It may occur in static vessels where there is presumably little if any hydrostatic pressure.

8. The diapedesis of erythrocytes, on the other hand, is a passive process depending upon intravascular pressure. Its occurrence is greatly exaggerated in areas in which intravascular pressure becomes elevated. Such elevations occur as the result of proximal arteriolar dilatation and distal occlusion of vessels.

9. Once they have reached the extravascular tissues the leucocytes move about more or less at random, apparently uninfluenced by any compelling chemotactic force. Their resultant migration, however, is toward the site of injury around which they eventually tend to congregate.

10. The histiocytes normally present in the connective tissue appear to play no role in the type of acute inflammatory reaction produced in these experiments.