Because thrombin aggregates afibrinogenemic platelets and platelets from patients with the gray platelet syndrome and because antibodies to fibrinogen inhibit thrombin-induced aggregation only at low concentrations of thrombin, the role of fibrinogen in the formation of thrombin-induced aggregates was investigated further with human platelets washed and resuspended in Tyrode-albumin solution containing apyrase, either with or without added Ca2+ (2 mmol/L). Samples for immunocytochemical assessment of fibrinogen distribution were taken at several times (up to five minutes) after aggregation induced by 0.5 U/mL of thrombin. Glutaraldehyde-fixed samples were embedded in Lowicryl K4M, sectioned, incubated with goat antihuman fibrinogen, washed, reacted with gold-labeled antigoat IgG, and prepared for electron microscopy. By 10 seconds, small aggregates formed, and granules were centralized; alpha granules were heavily labeled with immunogold, but the platelet surface was not. As large aggregates formed, granule swelling or fusion occurred, and in some areas granule material seemed to be in contact with the exterior. In these experiments with no added fibrinogen, there were some clusters of gold particles on the platelet surfaces remote from sites of granule discharge, but there were large areas where platelets were in close contact with little or no fibrinogen detectable between them. No fibrin was visible up to five minutes after the addition of thrombin, which indicated that fibrinogen from the granules does not readily become available for fibrin formation in the ambient fluid. Similar results were obtained in media with and without added Ca2+. Thus at least some aggregation in response to thrombin can occur without the participation of released fibrinogen, and much of the granule fibrinogen appears to remain localized at sites where granules fuse with the plasma membrane or the open canalicular system. Incubation of unstirred samples with thrombin for ten minutes resulted in the formation of small aggregates, extensive gold label in regions connected to the exterior of the platelets, but very little gold labeling of the platelet membrane and no visible fibrin formation. When the platelets were aggregated in the presence of external fibrinogen, the morphological changes within the platelets were the same, but fibrinogen rapidly became associated with the entire platelet surface, and visible fibrin formed within 30 seconds in the medium containing 2 mmol/L Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS).