Membrane lymphotoxin (LT) is a heterotrimer LTα1β2 , and its production depends on two genes. Northern blotting was employed in studying their transcription in human 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 to be similarly regulated in several cell lines and in blood lymphocytes. Activation of the LTα gene was associated with induction of transcription factor NF-κB (p50/p65) upon cell treatment with PMA. On evidence of RT–PCR, two transcripts of the LTβ gene were present in equimolar amounts in all lymphoid cells. A product of alternative splicing contained an open reading frame coding for the cytoplasmic portion of LTβ.