1. A method is presented for measuring the degree to which insulin antibodies in one antiserum react with an insoluble insulin complex saturated with antibodies from a different antiserum.

2. Many rabbits produce antibodies which bind to portions of the insulin molecule to which antibodies from guinea pigs or other rabbits cannot bind.

3. Occasional guinea pigs produce antibodies which bind to portions of the insulin molecule to which antibodies from rabbits or other guinea pigs cannot bind.

4. Studies with labeled antisera and repeated incubations of test antisera with antibody insulin complexes demonstrate the individual antibody variations to be due to antibodies directed to different determinants and not to dissociation of antibodies from the same determinant on the insulin molecule.

5. More than one antibody molecule can simultaneously bind to an insulin molecule.

6. Insulin has a multiplicity of antigenic determinants.

7. The relationship between antigenic determinants, insulin antibodies, and neutralization of insulin by antisera is discussed.

8. The determinants to which insulin antibodies are directed appear to be characteristic for the individual rabbit or guinea pig immunized. It is postulated therefore that genetic factors direct antibody production toward specific determinants when insulin is the antigen.

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