Exocrine γA globulin was isolated from human colostrum in a pure state. Measurement of s020,w and d20,w indicated a MW of about 393,000. Dissociation of the subunits of this 11.7 S molecule was effected by reduction and alkylation, with or without treatment with acid or sodium dodecyl sulfate. The results suggested that the 11.7 S exocrine γA molecule comprises two molecules of monomer γA globulin and one of secretory piece (SP) and is stabilized by both disulfide and noncovalent bonds. A protein was identified in normal colostrum with the same antigenic and size characteristics as the SP dissociated from exocrine γA globulin. The MW of SP was estimated to be 76,000 on the basis of gel filtration data.