A serum protein that is present in normal rat serum, and can be detected if serum is fractionated on polyacrylamide gel electrophoresis, disappears from the serum of hepatoma-bearing animals as the hepatomas increase in size. The protein has been purified and its molecular weight has been determined. It binds hemoglobin and could be a haptoglobin or hemopexin. The protein also disappears from the serum of rats kept for 3 weeks on a low protein diet. Such rats support growth of hepatomas poorly. It is postulated that the protein is preferentially used by hepatoma cells as a growth factor or surface effector.