Objectives: The objectives of this study were to determine whether there are differences in the electrophoretic profiles of plasma proteins from lean and obese rats and to identify a protein that was found to be more abundant in the plasma of obese rats.

Research Methods and Procedures: Plasma proteins from lean and obese Zucker fa and LA/N/fa^frats were separated by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. The identity of a band that was differentially expressed was determined by amino acid sequencing and Western blot analysis.

Results: A band migrating approximately the same distance as the 116 kDa molecular weight marker was more prominent in plasma from obese rats than in plasma of lean rats. Partial sequencing of the peptide revealed that 17 of the first 18 amino acids at the amino terminus were identical with the corresponding residues in the α‐chain of complement component C3. Western blot analysis confirmed the identity of the peptide as complement component C3. Complement C3 activity was measured using a hemolytic assay to determine whether there was a corresponding increase in the biological activity of this component in the serum of obese rats. Serum from obese rats was found to have 1.8 times as much complement component C3 activity as serum from lean rats.

Discussion: Elevated levels of complement C3 in genetically obese rats may be relevant because increased amounts of C3 could serve as a reservoir from which increased amounts of acylation stimulating protein, a cleavage product of complement C3, could be produced.