A recombinant tandem of 'chimeric' mouse/human immunoglobulin (Ig) genes was constructed and inserted into plasmid pGEM 1 under the control of the T7 bacteriophage RNA polymerase promoter. Lymphoid (Sp2/0) and non-lymphoid (CHO) cell lines used for transfection contained in their genomes a semisynthetic gene of T7 RNA polymerase and steadily expressed this enzyme. It was shown for the first time that a stable polycystronic transcription of the Ig gene tandem occurs under the control of a single T7 phage promoter, both in lymphoid and non-lymphoid cells. Synthesis of κ-light and -heavy Ig chains and functionally active antibodies was observed in the above-mentioned transfected cell lines.