An evaluation was made of the frequency of alpha (1 replaced by 3)-responsive, primary B precursors and their reactivity profiles in Igh-1al (al) and Igh-1b (beta) allogroup mice, of dissimilar responsiveness as to quantity of the specific serum antibody produced, its light chain isotype, and its variable-region, ligand-modifiable antigenicity, to alpha (1 replaced by 3) glucosyls in dextran B-1355. al mice, exhibited highly alpha 1-specific serum antibody, with relatedness to the idiotypes (Id) shared by three BALB/c, alpha (1 replaced by 3)-specific, gamma 1 myeloma proteins, possess specific primary splenic B precursors at a frequency of 1/5.5-7.3 x 10(4) cells; beta mice, exhibiting low, k serum, Id- antibody, possess B splenic-specific primary precursors at a frequency of 1/1.0-1.4 x 10(5) cells. A radioimmunoassay analysis showed correlations between the isotype profiles of the serum antibody response and of the primary precursors, namely mu gamma 1 in the al and mu kappa in the beta allogroups, but also delineated minor primary mu gamma lambda 1 and mu kappa precursors in al and mu gamma kappa and mu gamma 1 in b, at frequencies of 1/3 x 10(5) to 1/6.5 x 10(6). The al, gamma 1 precursors exhibited defined reactivity patterns for 3-mono-O-3,6-di-O-glucosyls and non-reducing end groups in the B-1355. The beta, kappa precursors recognized the sequential residues.