NF-KB is a nuclear protein, found only in cells that transcribe immunoglobulin light chain genes, that interacts with a defined site in the K immunoglobulin enhancer. This protein can be induced in pre-B cells by stimulation with bacterial lipopolysaccharlde (LPS). The induction involves a posttranslational activation, and the combined action of LPS and cycloheximide causes a superinduction. An active phorbol ester also induces this factor, and with kinetics more rapid than those for LPS stimulation. Phorbol-ester-mediated induction of NF-KB was observed in a T cell line (Jurkat) and a nonlymphoid cell line (HeLa), and is therefore not restricted to 5lymphoid cells. We interpret these results to indicate that factors that control transcrip tion of specific genes in specific cells may be activated by posttranslatlonal modification of precursor factors present more widely.