Summary 1. Proteins detectable by paper electrophoresis of concentrated bile were classified into A-, B-, C-, D- and E-fractions. 2. The A-fraction migrated more rapidly than any protein fractions in serum. 3. The Bfraction was similar to serum albumin as evidenced by its similar mobility to serum albumin and affinity for bile pigments and bromphenol blue. 4. C- and D-fractions had mobilities similar to serum globulins. 5. The E-fraction was probably a denatured A-fraction. and became prominent only after concentration of bile, associated with a decrease in the A-fraction. 6. Normal and pathological human bile, the bile of sheep and dog contain all these protein fractions. 7. Values of bile protein fractions varied considerably according to species and type of bile.