Interactions between platelets and fibrin have been visualized by phase contrast, epifluorescence, and scanning electron microscope examination of clots formed with dansylcadaverine-labeled fibrin and gel-filtered platelets. After thrombin activation, the platelets appeared as fluorescent aggregates with bridging strands of fibrin; formaldehyde- fixed platelets were not fluorescent under the same experimental conditions. Scanning electron micrographs demonstrated that thrombin- activated cells had numerous pseudopods to which the fibrin strands adhered; fixed platelets exhibited a smooth discoid appearance and did not interact with the clot. Platelets trapped in clots formed with Batroxobin (Pentapharm) (platelets are not activated by Batroxobin as confirmed by light-scattering aggregometry measurements) remained as nonfluorescent, discoid cells, whereas platelets first activated by adenosine diphosphate formed brightly fluorescent aggregates. Light- scattering data of thrombin activation (0.2 U/mL) indicated that preincubation of platelets with 0.1 mmol/L prostaglandin E1 (PGE1) prior to addition of thrombin decreased the extent and rate of platelet shape change and resulted in 100-fold slower aggregation. Clots formed in the presence of PGE1 revealed decreased fluorescence intensity and fewer platelet-fibrin contacts. Gly-Pro-Arg-Pro, which blocks fibrinogen binding and fibrin assembly, was also effective in blocking platelet-fibrin interactions. These results indicate that platelet activation is a prerequisite for attachment of platelets to fibrin.