In pre-B cells, the earliest identifiable stage of B cell differentiation, there is an asynchrony of immunoglobulin chain expression in that mu heavy chains are synthesized in the absence of light chain synthesis. These mu chains largely remain intracellular and are degraded. Here we demonstrate that a fraction of mu chains in human pre-B cell lines can reach the surface in association with three pre-B-specific proteins with relative molecular masses of 22, 18, and 16 kd, which we term collectively the pseudo-light chain complex, psi L. This association generates a multimeric complex, mu 2-psi L. Two of the psi L proteins (22 and 16 kd) are lambda-immunoreactive and form disulfide bonds with mu chains, suggesting that they are closely related to conventional lambda light chains. The 18 kd psi L species is a non-covalently-associated member of the complex. The expression of mu-psi L complexes on the surface of pre-B cells could have a functional role in the control of pre-B growth and differentiation by the hematopoietic microenvironment.