In human pre-B cells, the mu chain is associated with a surrogate light chain composed of the lambda-like and Vpre-B gene products. This pre-B cell receptor presumably triggers early steps of B cell differentiation, We have determined the NH2-terminal amino acid sequence of the lambda-like chain, showing that the mature chain results from the cleavage of a leader segment of 44 residues, leaving a polypeptide of 169 amino acids having partial features of the Ig light chain domains, with the exception of the first 50 amino acid NH2-terminal region. We have completed the nucleotide sequence of the Vpre-B gene, which appears to contain 126 residues in its mature form of which the 24 COOH-terminal portion was not Ig-related. Analysis of transfectants has provided direct evidence that lambda-like and Vpre-B chains assemble together even in the absence of heavy chain, prompting the search for a structural basis of this interaction. Comparison with the domain organization of the regular Ig lambda chain suggests that most of the psi L chain can be accommodated within a CL-VL-like structure, with an extra "subdomain" contributed by the non-Ig-like portions of both the lambda-like and Vpre-B polypeptides.