Abstract:Myeloma M 104E (IgM, Λ1) with specificity for the α(1 → 3) glucosidic linkage of dextran B 1355 S (Dex) carries two idiotopes (Id) as defined by isogenic anti‐idiotype mAb. The public Id is not influenced by two amino acid replacements in CDR 2 nor by an alternative D region sequence. It is shared by all or nearly all humoral antibodies in the primary immune response against Dex of mice carrying haplotype Igha. The private Id‐5 appears to depend on the integrity of the VH germ‐line sequence and on the particular M 104E D region sequence. It is present on a highly fluctuating but usually small fraction of primary anti‐Dex antibody. We report here that this situation is changed when mice are treated with Cobra venom anti‐complement factor (CVF) after birth and thereby were deprived of complement for the first two weeks of life. When immunized with Dex as adults the majority of anti‐Dex Ab carried the M 104E Id‐5. Thus, humoral antibody in CVF‐treated animals resembled the Ly‐1+ anti‐Dex precursor B cell population in the peritoneal cavity, while anti‐Dex Ab in animals not treated with CVF more closely corresponded to the Ly‐1− precursor B cell pool in spleen (H.‐P. Lehmann and G. Lehle, Eur. J. Immunol. 1991. 21: 1201).