Lymphatic clearance of radioactive sulfur colloid is measured as a function of externally applied pressure on the hind limb of mongrel dogs. A dead weight device is placed over the site of subcutaneous injection. A solid state Si (Li) is placed into a slot at the bottom of the device to continuously record activity of the tracer. An exponential decrease in activity is modeled as a dual decay resulting from both tracer half life decay and lymphatic clearance of the tagged sulfur colloid. External pressure is seen to enhance lymph clearance until a critical closing pressure is reached, whereupon the vessel collapses and lymph flow is drastically reduced. A closing pressure of 60 mmHg is observed for several experiments. Lymph flow per tissue volume is seen to rise from a mean of 0.324 ml/hr/ml for uncompressed tissue to 0.96 ml/hr/ml for fully enhanced flow in other experiments at 60 mmHg. Results at a pressure of 75 mmHg show almost no lymph clearance suggesting complete vessel closure.