Abstract The possibility that idiotype dominance may be associated with increased affinity for hapten was investigated in the murine A/J anti- p-azophenylarsonate (Ars) response. Fluorescence quenching of 14 Ars-binding hybridoma proteins by Ars-tyrosine was measured and Ka calculated using computerassisted curve fitting. There was a 200-fold range in Ka for idiotype-positive hybridoma proteins, with 2 IgM hybridoma proteins being near the median. No clear difference in Ka was apparent between idiotype-positive (Id +) and idiotype-negative (Id −) hybridoma proteins. Ka was measured by fluorescence quenching on affinity-purified anti-Ars antibodies from 6 conventional antisera; there was no difference between Id + and Id − (idiotype suppressed) sera. The affinities of the hybridoma proteins were correlated with the ratio of binding to Ars 36-BSA and Ars 10-BSA by direct radioimmunoassay. With this calibration, functional affinities of Ars-immune sera could be determined from relative binding ratios without the need for prior affinity purification. This was done for 18 Ars-immune sera, and again there was no clear difference between Id + and Id − sera. Studies from this laboratory have identified the amino acid sequence of a hybridoma protein which corresponds to the germ line DNA sequence for the cross-reactive idiotype family. The present study shows that the protein directly encoded by the germ line gene has low affinity for hapten suggesting that somatic diversification operating on the germ line sequence can produce antibodies with increased affinity for hapten within the cross-reactive idiotype family. The present study also suggests that affinity is not the driving force behind idiotype dominance of the Ars-immune response.