A suspension of Chang''s human hepatoma cells was inoculated into the tail vein of rats. Rats were killed at different intervals ranging from 30 s to 3 h. Peroxidase-labeled goat anti-rabbit gamma globulin was applied to the tissue by an indirect method. Peroxidase-positive material was always found between the tumor cell and the endothelial lining and at the center of the ''platelet tail'' as early as 30 s. Polymerized fibrin fibers 1st appeared at 15 min and more at 30 min. Fibrin precursors may act as a ''glue'' for the tumor cell arrest.