The viral Tax protein, which is encoded by human T-cell leukaemia virus HTLV-I, activates nuclear translocation of the NF-kappa B/Rel transcription factors and relieves cytoplasmic sequestration of RelA and Rel by heterodimerization with NF-kappa B1/p1O5 (refs 1,2). Proteolytic maturation of this precursor protein is performed by the proteasome complex. Here we show that Tax binds specifically to two subunits of the 20S proteasome, HsN3 and HC9. This interaction is weakened with HsN3 and lost for HC9 when a mutant of Tax is substituted that is selectively defective for NF-kappa B activation. Immunoprecipitation shows that p1O5 binds weakly to HC9 and that this interaction is reinforced by Tax. No bridging function of Tax between p1O5 and HsN3 was observed. From these results, we propose that Tax accelerates the proteolytic maturation of P105 by favouring its anchorage to the proteasome.