BALB/c mice immunized with bacterial levan (BL) produce an immune response that fails to generate antibody expressing the idiotype (Id) of the beta (2 leads to 6) fructosan-binding myeloma protein ABPC 48 (A48). Pretreatment of newborn BALB/c mice (at 1 d of age) with 0.01-10 microgram of affinity purified BALB/c anti-A48 Id antibody followed by immunization with BL 1-2 mo later produces an anti-BL response that expresses the A48 Id. This shows that A48 Id+ anti-BL clones belong to a normally silent fraction of the anti-BL repertoire. The activation of A48 Id+ anti-BL clones anti-A48 Id antibody is specific because the pretreatment of newborn mice with anti-MOPC 384 Id antibody, followed by immunization with BL, does not lead to its activation. Moreover, pretreatment of mice with anti-A48 Id antibody does not alter the MOPC 460 Id+ component of the anti-TNP response. It is also important to note that the activation of the A48 Id+ clone in pretreated mice requires subsequent immunization with BL.