A method for producing protein-iodination-grade 123I suitable for use with a compact biomedical cyclotron is reported. The preparation of highly iodinated fibrinogen (25 123I atoms/molecule) is described, and its successful use as a thrombus-imaging agent in experimental animals is reported. This new agent clears from the blood faster than conventional radio-iodinated fibrinogen and gives higher thrombus-to-blood activity ratios. Thus, the detection of deep-vein thrombi in areas of large blood pool is enhanced, and images can be obtained sooner after administration of the radiopharmaceutical. Induced 4-8 h old femoral-vein thrombi in dogs can be well visualized with a scintillation camera as early as 4 h and as late as 15 h after administration of 1 mCi of 123I-labeled highly iodinated fibrinogen.