The fine structure of vascular permeability was studied on the dorsal skin of rabbits by a single intradermal injection of the extract, cationic protein, obtained from a-granules of bovine platelets after separation as subcellular fractions. As early as 1-0 min., the ferritin particle, used as a tracer, frequently leaked out through the junctional gaps of postcapillary venular endothelium into the perivascular connective tissue. At this period, however, mild exudation also occurred even in the

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... ites of small veins and capillaries. At 3-5 min., inflammatory cells already infiltrated into the vessel wall or surrounding tissue. With the lapse of time, the inflammatory cells increased in number and strands of fibrin appeared in the connective tissue by 3 hours. Neither exudation of f erritin particles nor cellular migration appeared in the vessels of control rabbits given isotonic phosphate buffered saline. Such f erritin exuda tion and cell migration from the vessels were scarcely inhibited in the animals pretreated with histamine H1-and H2-receptor antagonists before the injection of extract. In conclusion, the a-granule extract from bovine platelets has vascular permeability factor(s) as described in that from human platelets and causes early permeability response.