The VpreB and λ5 proteins, together with Igμ-H chains, form precursor BCRs (preBCRs). We established λ5−/−/VpreB1−/−/VpreB2−/− Abelson virus-transformed cell lines and reconstituted these cells with λ5 and VpreB in wild-type form or with a deleted non-Ig part. Whenever preBCRs had the non-Ig part of λ5 deleted, surface deposition was increased, whereas deletion of VpreB non-Ig part decreased it. The levels of phosphorylation of Syk, SLP65, or PLC-γ2, and of Ca 2+ mobilization from intracellular stores, stimulated by μH chain crosslinking Ab were dependent on the levels of surface-bound preBCRs. It appears that VpreB probes the fitness of newly generated VH domains of IgH chains for later pairing with IgL chains, and its non-Ig part fixes the preBCRs on the surface. By contrast, the non-Ig part of λ5 crosslinks preBCRs for downregulation and stimulation.