1. Serum proteins of upstream migrants of Petromyzon marinus were separated electrophoretically on polyacrylamide and agarose as well as by crossed immunoelectrophoresis using anti-lamprey serum antiserum. 2. Some constituents of four of the five major bands separated by electrophoresis on polyacrylamide were located in the agarose and crossed-immunoelectrophoretic patterns. 3. Two serum proteins, SDS-1 and CB-III, were isolated. SDS-1 was found to be a glycoprotein; CB-III may be a lipoprotein. 4. Monospecific antisera were produced against SDS-1 and CB-III and were used in crossed immunoelectrophoresis to analyze lamprey serum. This method revealed that electrophoretically different forms of these two proteins exist in serum. 5. Immunoelectrophoretic methods demonstrated that three of the major bands produced by polyacrylamide gel electrophoresis of whole serum contained more than a single protein.