This study was designed to determine the transport of subcutaneously injected viral-size colloid particles into the lymph and the vascular system in the hind leg of the dog. Transport of two colloid particles, with average size approximately 1 and 0.41 microm, respectively, and with and without leg rotation, was tested. Leg rotation serves to enhance the lymph flow rates. The right femoral vein, lymph vessel, and left femoral artery were cannulated while the animal was under anesthesia, and samples were collected at regular intervals after subcutaneous injection of the particles at the right knee level. The number of particles in the samples were counted under fluorescence microscopy by using a hemocytometer. With and without leg rotation, both particle sets were rapidly taken up into the venous blood and into the lymph fluid. The number of particles carried away from the injection site within the first 5 min was <5% of the injected pool. Particles were also seen in arterial blood samples; this suggests reflow and a prolonged residence time in the blood. These results show that particles the size of viruses are rapidly taken up into the lymphatics and blood vessels after subcutaneous deposition.