SUMMARYStructural studies of T‐cell immunoglobulin light chains have been carried out in order to ascertain whether they possess a unique structure or if they resemble standard kappa or lambda isotypes. T‐cell immunoglobulin was isolated from125I‐labelled culture medium of monoclonal, continuously cultured T‐lymphoma cells, and the purified125I‐labelled light chains were subjected to either cleavage by cyanogen bromide or digestion with trypsin. These peptides were then resolved and compared with those derived from125I‐ and131I‐labelled murine kappa and lambda chains in mixed label experiments, by means of polyacrylamide gel electrophoresis in sodium dodecyl sulfate‐containing buffers and ion exchange chromatography. The profiles obtained suggested that T‐lymphoma immunoglobulin light chains are immunoglobulin polypeptides and most closely resemble kappa chains. These results support available antigenic and mRNA hybridization data, which also suggest that T cells bear kappa