The pre-B cell receptor (preBCR), composed of μ immunoglobulin (Ig) and surrogate light chains, signals large 'preB-II' cells to proliferate in the apparent absence of ligands or cooperating cells. We deleted the N-terminal, nonimmunoglobulin (nonlg) portion of λ5, or mutated seven arginine residues in it to serine residues. PreBCRs with such mutant λ5 proteins showed increased cell surface representation and a diminished rate of aggregation and internalization. Tyrosine phosphorylation of preBCR complexes containing mutant λ5 proteins was abolished. These results indicate that the nonIg portion of λ5, and the seven arginine residues in it, are needed for signal transduction, and that signaling could be cell autonomous. We propose two models to explain the apparently constitutive, ligand-independent signal-transducing capacity of the preBCR.