Interactions between platelets and fibrin are important in hemostasis but often confused with platelet-fibrinogen interactions. A stirred mixture of solubilized fibrin and washed platelets at neutral pH range showed drastic reduction in turbidity and concomitant platelet adhesion onto newly formed fibrin strands. This platelet-fibrin interaction did not require platelet activation nor did it cause platelet aggregation. A device consisting of a parallel-plate flow chamber mounted on a fluorescence microscope has been constructed to allow direct visualization and recording of platelet-fibrin interaction under flow conditions. Platelets in whole blood adhered to the fibrin-coated portion but not to the uncoated portion of the flow chamber. Slow motion playback of video tapes indicated that the adhesion phenomenon was a dynamic process that involved attaching, detaching, relocation and transient contact. The fibrin coating influenced platelet adhesion both by increasing the number of cells making short-term attachments to the surface and by increasing the duration of cells attached to the surface. These observations provided basic characteristics of platelet-fibrin interaction.