Bovine milk sulfhydryl oxidase was prepared by transient covalent affinity chromatography on a cysteinylsuccinamidopropyl glass matrix. The homogeneity of this preparation was examined by immunofixation electrofocusing, SDS-PAGE, and size exclusion chromatography in 6 M guanidinium chloride. The major contaminant of this preparation, prepared from whey diafiltered with a 1000 kdal exclusion limit membrane, was IgG. Immunoglobulin M could not be detected. Presence of IgG in these sulfhydryl oxidase preparations suggests an interaction between IgG and either skim milk membrane vesicles or sulfhydryl oxidase. © 1990, American Dairy Science Association. All rights reserved.