Edinburgh EHI 2QN, Scotland

ies against human fibrinogen and the platelet-specific proteins platelet factor 4 and beta thromboglobulin were used to localize these antigens in thin and ultra-thin frozen sections of mildly fixed, washed human blood platelets. By immunofluorescent double-labeling experiments the distribution of fibrinogen was com-pared to that of platelet factor 4 and beta thrombo-globulin. All three antigens occurred in virtually all platelets and showed an identical, dotlike distribution. For immunoelectron microscopy we used protein A-colloidal gold on ultrathin frozen sections to visualize the specific reaction indirectly. The staining for plate-let factor 4, beta thromboglobulin, and fibrinogen lo-calized exclusively over a-granules of washed platelets. Within the granules, platelet factor 4 was localized preferentially in the electron dense, a-granule nu-cleoid, whereas fibrinogen was more predominant in the electron-lucent granule periphery. Beta throm-boglobulin localization did not show a preferential in-tragranular distribution