Specific plasma proteins were labeled with 131I, and their half-lives in lactating and nonlactating mice were determined. The proteins included mouse IgF and IgM, mouse, bovine, rabbit, and human IgG, human serum and salivary IgA, human transferrin and albumin, and mouse and human immunoglobin light chains. The rates at which these proteins are transferred across the mammary gland in mice suckling their young were calculated: transmammary protein transfer was found to be highly selective. The G-class immunoglobulins readily traversed the mammary gland, the rate being dependent on the IgG load presented to the transport system. Immunoglobulin lambda-chains and human serum IgA crossed the mammary gland even more rapidly than did the G immunoglobulins. Human IgA without secretory piece was transported into milk more rapidly than IgA with secretory piece attached. Protein transport across the mouse mammary gland was found to be similar to and yet different from protein transport across either the suckling mouse intestine or the mouse placenta as to which proteins crossed and at what relative rates.