In murine immunoglobulins, the κ chain is the major light chain isotype, as exemplified by the small amount of λ immunoglobulins present in normal sera, the rare occurrence of λ plasmacytomas and the preponderance of κ light chains in induced antibodies. Notable exceptions to this are the response to α(1→3) dextran1 in BALB/c mice and the heteroclytic antibodies raised against the nitrophenol hapten in C57BL/6 mice2. In the former response, the expression of idiotypes in the hypervariable regions of the heavy chain3 is characteristically restricted. We have now analysed the B-cell precursors for the α(1→3) linkage-specific response induced by T-dependent and T-independent dextran antigens with respect to light chain isotype and heavy chain idiotype expression. Our findings clearly demonstrate that the two forms of dextran antigen trigger different B-cell precursor subpopulations. Furthermore, each individual animal appears to possess a different repertoire of idiotype-committed precursors. This contrasts with the response of BALB/c mice to phosphorylcholine (PC) where most anti-PC antibodies and precursors respond with the dominant expression of the T15 idiotype family, regardless of whether T-dependent or T-independent PC antigens are used4. © 1981 Nature Publishing Group.