Precipitation of serum proteins of the house mouse, Mus musculus, by trichloroacetic acid and redissolving part of the precipitate in ethanol proved to be a very efficient procedure for isolating mouse serum albumin. Electrophoretic and immunoelectrophoretic analyses showed only albumin to be dissolved at a detectable level in the ethanol. Electrophoretic studies also showed that isolated albumin underwent polymerization with the polymers being readily separated by exculsion chromatography on Sephadex G-150. Comparisons of such physicochemical characteristics of the albumin monomers, as extinction coefficient (5.2), sedimentation coefficient (4.5 S), molecular weight (67 000), free sulfhydral groups per molecule (0.6) and amino acid composition with albumins of other mammals showed strong similarity. Finally, a comparison was made between the common mouse albumin or albumin A, and a variant, albumin C. The two types of albumin molecules were found very similar in all respcets except electrophoretic migration.