The transcription factor NF-kappa B appears to play an important role in immunoglobulin gene expression and lymphokine production, and may play a role in primary B cell activation. Constitutive nuclear expression of NF-kappa B has been found in all mature B cell lines with the notable exception of the murine plasmacytoma, S107. We report herein that S107 cells express cytoplasmic kappa B-binding material detected by electrophoretic mobility shift assay that by several criteria represents authentic NF-kappa B. Despite the presence of cytoplasmic NF-kappa B, several stimuli known to induce nuclear translocation of NF-kappa B failed to do so in S107 cells, including: the PKC agonist, PMA; the protein synthesis inhibitor, cycloheximide; and LPS. Transfection of S107 cells with a kappa B-CAT reporter gene construct confirmed the absence of functional activity. Importantly, a global failure of nuclear transcription factor expression was ruled out by the ability of PMA to induce nuclear expression of another trans-acting factor, AP-1. Thus, rather than lacking NF-kappa B altogether, S107 cells manifest disordered regulation of NF-kappa B in which cytoplasmic material is incapable of translocation to the nucleus. While Northern analysis failed to reveal a gross defect in the mRNA coding for the DNA binding subunit of NF-kappa B, UV-photo-cross-linking followed by denaturing gel electrophoresis demonstrated the presence of a cytoplasmic kappa B-binding protein of abnormally elevated molecular size. This finding suggests that the abnormal regulation of NF-kappa B in S107 cells is associated with the appearance of an unusual kappa B-binding molecule.