The ontogeny of BALB/c B cell repertoire with specificity for α1->3 dextran (DEX) was examined by using monoclonal anti-idiotype antibodies (MAID). Anti-DEX B cell precursors were absent from donor mice that were less than 5 days old. Precursors were first detected in mice from 5 to 11 days of age, but at a very low frequency of <1 in 108. In older mice, the frequency of anti-DEX precursors increased to approximately adult levels. Seventy-five percent of splenic foci from 5 to 11-day-old donors expressed IgA, and 70% expressed both IgM and IgA. The frequency of DEX-positive foci containing secreted IgA and IgM-IgA decreased as the age of the donors increased. The frequency of IgM-secreting foci remained constant at about 90% of DEX-positive foci regardless of donor age. The frequency of the MAID-defined cross-reactive idiotype CD3-2 and the EB3-16 idiotype changed very little in frequency with age, whereas the EB3-7 and LA4-8 idiotypes increased in frequency as donor age increased. Conversely, the SJL18-1 idiotype that was predominant at days 5 to 11 decreased in frequency relative to total DEX-positive foci as the age of the donors increased. The ratio of M104E-like and J558-like molecules from neonatal B cell precursors is reversed from that expressed by adult B cell precursors, and may reflect the preferential expansion of precursors bearing certain idiotypes by environmental antigens.