Summary: The nurse shark was shown to produce antibodies to bovine serum albumin detectable by passive hemagglutination. The sera of both normal and immunized nurse sharks contained high levels (∼50% of the optical density) of immunoglobulins with sedimentation coefficients of approximately 19 S and 7 S. These immunoglobulins were isolated and purified by diethylaminoethyl cellulose chromatography followed by gel filtration on Sephadex G 200. The immunoglobulins were antigenically identical when analyzed with rabbit antisera. They also had similar immunoelectrophoretic mobilities and hexose contents. Extensive reduction and alkylation dissociated both molecules into H (MW ∼70,000) and L (MW ∼22,000) polypeptide chains in proportions comparable to IgM. It was demonstrated that nurse and lemon sharks had antigenically related L-chains and dissimilar H-chains, thus suggesting a differential rate of evolutionary changes in immunoglobulin chain structure.