The mechanism of the clearance of circulating tin-protoporphyrin (Sn-PP), a competitive inhibitor of heme oxygenase in the degradation of heme to bilirubin, is unknown. Two serum proteins, albumin and hemopexin, which are instrumental in the delivery of iron-protoporphyrin (heme) to the liver, also bind metalloporphyrins with high affinity and may aid in targeting their tissue distribution. After intravenous injection of 1 mumol Sn-PP/kg, the serum concentration of hemopexin decreased in human subjects, rats, and rabbits within 24 hours to a similar extent (30% to 50%). This finding suggested that hemopexin may have a role in the tissue distribution of Sn-PP. However, when rats were injected with Sn-PP in saline solution or complexed with albumin, more Sn-PP was taken up by the liver and testes than when Sn-PP was complexed with hemopexin. These results indicate that hemopexin does not preferentially target Sn-PP to the liver and may not be the preferred vehicle for clearance of circulating Sn-PP.