Lewis rat antibodies raised by immunization with encephalitogenic peptide 68-88 guinea pig myelin basic protein were purified by affinity chromatography and used to immunize rabbits. After exhaustive absorption of the rabbit antisera to remove anti-rat immunoglobulin activity, the antisera retained activity against the immunogen, shown by the ability to block reaction of radioiodinated peptide with the active site of the rat anti-peptide antibodies. Intrastrain idiotypic cross-reactivity was assessed by testing the rabbit antisera against a panel of Lewis anti-peptide antibodies. Each anti-idiotypic antiserum displayed a unique pattern of reactivity with the panel. Similar tests in which a panel of anti-peptide antibodies raised in F344 rats was used demonstrated the presence of interstrain cross-reactive idiotopes. When seven rabbit anti-idiotypic antisera were tested by pretreatment of rats before challenge with encephalitogen for effect in vivo, five were without effect. Of the remaining two, one caused a slight suppression of disease; the other enhanced disease compared to control animals.