Milk proteins from BALB/c and C3H mice were characterized with respect to their electrophoretic migration in polyacrylamide gels under alkaline and acid conditions. The major casein and whey proteins from each strain migrated similarly under the conditions employed. Phosphoproteins were identified by staining with "Stains-all" and by changes in electrophoretic mobility and staining induced by prior treatment with phosphatase. Sialic acid-rich glycoproteins were identified by staining with periodic acid-Schiff and with "Stains-all" by prior treatment with neuraminidase to identify sialic acid as the acidic portion of the molecule. The two major whey proteins were characterized further by their migration in sodium dodecyl sulfate gels. One protein had the same mobility as mouse serum albumin. The other protein migrated with a mobility similar to that of bovine alpha-lactalbumin. The identity of the former protein was confirmed by its reaction with goat anti-mouse serum albumin in an immunodiffusion procedure, and the latter protein by its B protein activity in the lactose synthetase assay.