Heme oxygenase was purified to apparent homogeneity from pig spleen microsomes. The purified heme oxygenase showed an apparent molecular weight of 157,000±7,000 daltons when estimated by gel filtration On SDS-polyacrylamide gel electrophoresis, the heme oxygenase preparation gave a smgle protein band showing a minimum molecular weight of about 26,000 daltons. 1-leme oxygenase could readily bind with heme and the resulting heme complex gave an absorption maximum at 406 nm. The heme bound to the enzyme protein was found to be a good substrate for the heme oxygenase reaction