Aqueous solutions of lanthanum nitrate may be used as electron microscopic tracers in vivo to study vascular permeability in the experimental animal. However, with this technique the size of the tracer articles is not known. To gain information about the tracer size, we injected lanthanum nitrate into the blood circulation of living rabbits. The plasma obtained from such animals 30 min later, was studied with the electron microscope. The plasma contained an electron-dense material, readily visible in the electron microscope. A precipitate obtained after centrifugation of the whole blood to separate the cells, also contained the tracer. Lanthanum was found in large amounts in the fibrin clot obtained after treating the plasma with thrombin. The tracer was not detected in the "serum" (i.e. thrombin-treated plasma). The study indicates that ionic lanthanum injected into the blood circulation of living rabbits, is to a great extent bound to fibrinogen, and that the smallest possible size of the tracer is that of the fibrinogen molecule (m.w. 330,000). Larger particles are present as well.