Following injection of rabbit 125I-asialohemopexin, more than 90% of the protein-bound 125I was removed from the circulation of rabbits within 12 min. The amount of asialoprotein in the catabolic compartment reached a peak concentration (75 to 85%) 12 min after injection and was completely eliminated from this compartment within 2 hours. The degradation products were excreted into the urine, with 50 to 70% of the 125I eliminated during the first 24 hours and 90 to 95% excreted by 48 hours. Analysis of these data indicated an apparent first order rate constant for uptake of asialohemopexin of 0.32 min-1, for catabolism of 0.020 min-1 and for excretion of 0.054 to 0.093 hour-1. The plasma distribution curves of 125I-hemopexin, after the first 24 hours, showed essentially no difference. Both proteins were catabolized with an average T1/2 of 25 to 26 hours and a similar fractional catabolic rate. Simultaneous injection of heme and 125I-hemopexin resulted in rapid removal and catabolism of the protein. In contrast, injection of heme had little if any effect on the plasma radioactivity curve of photoinactivated 125I-hemopexin.