In pre-B cells, the earliest identiflable stage of B cell differentiation, there is an asynchrony of Immunoglobulin chain expression in that μ heavy chains are synthesized in the absence of light chain synthesis. These μ chains largely remain intracellular and are degraded. Here we demonstrate that a fraction of μ 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, ψL. This association generates a multimeric complex, μ2–ψL. Two of the ψL proteins (22 and 16 kd) are λ-Immunoreactive and form disulfide bonds with μ chains, suggesting that they are closely related to conventional λ light chains. The 18 kd ψL species is a non-covalently-assoclated member of the complex. The expression of μ–ψL complexes on the surface of pre-B cells could have a functional role in the control of pre-B growth and differentiation by the hematopoletic microenvironment.