Normal adult male mice of various strains excreted 0.6–3.1 mg of urinary protein N/day; females, considerably less. Electrophoretic measurements showed the protein to consist of three components, all having negative mobilities (–7.5 to –8.3 cm2/volt/sec x 10–5 at ph 8.5) greater than that of mouse serum albumin (–6.3 ± 0.1 cm2/ volt/sec. x 10–5 at ph 8.5). The most negative, amounting to approximately 32% of the total urinary protein, had an isoelectric ph of 4.3; the least negative, 4.5. During ultracentrifugation the urinary protein sedimented as a monodisperse system with a sedimentation coefficient of 2.28 S and a weight—average molecular weight of 17,800. Electrophoresis of mouse serum revealed a small fast-moving component which may be identical with the urinary protein.