We have cloned double stranded cDNA sequences encoding a mouse immunoglobulin light chain (L-321) into the PstI site of the β-lactamase gene of plasmid pBR322 by the oligo (dG)-oligo (dC) tailing procedure. Escherichia coli XI776 transformed by the recombinant plasmids were screened for the expression of L-321 antigenic determinants by a newly developed in situ radioimmunoassay. One out of seven transformants screened was found to synthesize an L-chain like protein. Each bacterial cell produces about 550 molecules of the L-chain sequence. Preferential segregation of the L-chain sequence into the periplasmic space suggests covalent attachment of the L-chain sequence to the N-terminal portion of β-lactamase. Restriction mapping of the plasmid DNA Isolated from the positive clone indicated the presence of a DNA sequence coding for the entire constant region and extending into the variable region for a length corresponding to about 40 amino acid residues. The orientation of the cloned cDNA with respect to the plasmid DNA is compatible with the formation of a fused β-lactamase-L-321 peptide.