The anti-2,4-dinitrophenyl (DNP) antibodies synthesized by suspensions of lymph node cells obtained at various intervals from rabbits that had been immunized with DNP-bovine γ-globulin increased progressively in their affinity for the dinitrophenyl determinant. This change accompanied and was apparently responsible for a similar change in the binding properties of anti-DNP antibodies isolated from the serum. The rate of change in affinity was related to the dose of immunogen: increasing the dose delayed the change.

The antibodies formed during a brief (5 hr) incubation in vitro were heterogeneous in their binding properties. Therefore, the mixing in the circulation of molecules synthesized at different times may contribute to, but is not alone responsible for, the heterogeneity in the serum antibodies.

Variability in binding did not appear to be related to heterogeneity in immunoglobulin class. Indeed, the variations in relative affinity occurred entirely within the γG-immunoglobulins.

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