The amino-terminal propeptide of type III procollagen (PIIINP, M(r) 42,000) is a promising marker for the formation of type III collagen of granulation tissue in experimental and clinical studies. The disposal kinetics of circulating PIIINP is, however, almost unknown. In conscious pigs with a thoracic duct-venous shunt, 125I-labeled PIIINP was injected intravenously. The initial distribution volume was 2.2 liters, which was 1.7 times the plasma volume (P < 0.01). The disappearance curve was three-phased, with an initial steep decline (t1/2 58 min), followed by two slower phases (t1/2 239 min and 289 h). Consecutive gel filtrations showed that the initial slope of the plasma disappearance curve corresponded to the plasma clearance of the intact PIIINP. The initial plasma clearance was 26.5 ml plasma/min, whereas the urinary clearance was 8.7 ml plasma/min (P < 0.01). The other components of the plasma disappearance curve originated from the formation and disappearance of a high and a low molecular weight (MW) fraction as part of the degradation of PIIINP. The high MW fraction (approximately M(r) 90,000) was similar to a previously described, but not further characterized, PIIINP immunoreactive component. The existence of the low MW fraction (approximately M(r) 20,000) has not been reported before. The lymphatic recirculation of intact PIIINP was rapid, and the lymph-serum ratio was almost constant within 1 h of injection. We conclude that the t1/2 of circulating PIIINP is 58 min, that PIIINP escapes the circulation very quickly, and that the degradation of PIIINP includes at least two intermediary steps.