In pre-B cells, the earliest Identifiable 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-spectfic 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 multlmerlc complex, μ2 -ΨL. Two of the ΨL proteins (22 and 16 kd) are λ-lmmunoreactive 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 hematopoietic microenvironment. © 1989, The Japanese Society tor Immunology. All rights reserved.