The appearance of an idiotypic specificity, present in anti-p-azophenylarsonate (anti-Ar) antibodies of all immunized A/J mice, ran be suppressed in adult mice by prior administration of an IgG fraction of rabbit antiidiotypic (anti-D) antiserum; anti-Ar antibodies arise but are of different idiotype. Prolonged suppression was observed in earlier experiments, but antigen was first administered to adult mice only 2 wk or 9 wk after anti-D antibodies; subsequent escape from idiotypic suppression could have been masked by the capture of antigen by large numbers of memory cells having receptors of a different idiotype. In the present experiments antigen was first administered at intervals up to 22 wk after the antiidiotypic antibody. Suppression was maintained for 6 wk in all mice and for 5 mo in about half the mice tested. It thus appears that suppression of idiotype is less reversible if antigen is administered soon after the antiidiotypic antibody. The data suggest that escape from suppression is attributable to the generation of new precursor cells rather than to reactivation of suppressed cells. The minimum dosage of antiidiotypic IgG required for effective suppression was about 2 mg. The subcutaneous or intraperitoneal routes of inoculation of antiidiotypic IgG were equally effective. When antiidiotypic antibody was administered 3 days after antigen no suppressive effects were observed. There was partial suppression when antiidiotypic antibody was injected on the same day as the antigen. Fab' and F(ab')2 fragments of antiidiotypic IgG had no suppressive effect. Quantitative measurements revealed no significant differences among control and suppressed mice with respect to total concentration of precipitable anti-Ar antibodies produced
