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The intracellular protozoan parasite Theileria parva causes a lymphoproliferative disease of T cells in cattle and uncontrolled lymphocyte proliferation in culture. We have identified and characterized in infected cells the transcriptional activator, NF-κB, whose recognition motifs have been identified in several gene enhancers important for lymphocyte-specific gene expression. NF-κB is normally constitutively activated in nuclear extracts derived from B cells and can be induced in T cells and nonlymphoid cells by phorbol esters. Theileria-infected lymphocytes contained constitutively high levels of activated NF-κB in nuclear fractions and inactive NF-κB in cytoplasmic fractions. The inactive cytoplasmic precursor could be activated by treatment of extracts with deoxycholate, which was shown previously to dissociate NF-κB from an inhibitor, IkB. Treatment of lymphocyte extracts with 3 mM GTP stimulated NF-κB binding to …