Membrane lymphotoxin (LT) is heterotrimer LT alpha 1 beta 2, and its production depends on two genes. Northern blotting was employed in studying their transcription in B- and T-lymphoma cell lines and in peripheral blood lymphocytes before and after induction with phorbol myristate acetate (PMA). Transcription of either gene proved similarly regulated in several cell lines and in blood lymphocytes. Activation of the LT alpha gene was associated with induction of transcription factor NF-kappa B (p50/p65) upon cell treatment with PMA. On evidence of RT-PCR, two transcripts of the LT beta gene occurred in equimolar amounts in all lymphoid cells. A product of alternative splicing contained an open reading frame coding for the cytoplasmic portion of LT beta.