The constant region of the human Ig lambda locus consists of seven tandemly organized J-C gene segments. Although it has been established that the J-C lambda 1, J-C lambda 2, J-C lambda 3, and J-C lambda 7 gene segments are functional, and code for the four distinct Ig lambda isotypes found in human serum, the J-C lambda 4, J-C lambda 5, and J-C lambda 6 gene segments are generally considered to be pseudogenes. Although one example of a functional J-C lambda 6 gene segment has been documented, in the majority of cases, J-C lambda 6 is rendered nonfunctional by virtue of a single duplication of four nucleotides, creating a premature translational arrest. We show here that rearrangements to the J-C lambda 6 gene segment do occur, and that such a rearrangement encodes an Ig lambda protein that lacks the terminal end of the constant region. We also show that this truncated protein is expressed on the surface with the IgH chain, creating an unusual surface Ig (sIg) receptor (sIg delta CL). Cells that express this receptor on the surface do so at significantly reduced levels compared with clonally related variants, which express sIg receptors with conventional Ig lambda L chains. However, the effects of sIg cross-linking on tyrosine phosphorylation and surface expression of the CD25 and CD71 Ags are similar in cells that express conventional sIg receptors and in those that express sIg delta CL receptors, suggesting that the latter could possibly function as an Ag receptor.