The derived amino acid sequences of human T-cell receptor beta chain shows significant homology to lambda light chains of immunoglobulins in its variable, joining, and constant regions. We assessed the cross-reactivity between Tcr beta chains and immunoglobulin light chains by determining the capacity of rabbit antisera to human or murine immunoglobulins to react to a synthesized set of nested, overlapping 16-mer peptides corresponding to the VDJC sequence of the Tcr beta chain YT35. The observed reactivities were consistent with homologies to lambda and kappa light chains, the strongest reactivity being with a peptide that corresponds to the "switch peptide" of light chains, as assessed by ELISA binding and competitive inhibitions assays. Other regions reactive with anti-light chain sera corresponded to CDR1 and Fr3 segments of the variable domain and a segment of the constant region predicted to loop out of the tight globular structure. The peptide immunochemical results, together with the identification of specific regions of sequence correspondence between Tcr beta and the characterized lambda light chain Mcg, allowed us to develop a 3-dimensional model of the beta chain consistent with its role in antigen recognition.