RelB belongs to the family of Rel-related proteins, dimers of which determine NF-kappa B activity. The murine RelB protein has been reported to be a dimerizing partner in kappa B-binding complexes which are capable of transactivation. On the other hand, the I-Rel protein, the presumed human homolog of RelB, was proposed to be an inhibitor whose presence in dimeric complexes interfered with their kappa B binding and therefore interfered also with transactivation. We demonstrate that human RelB (I-Rel) forms with p50 and p52 (p50B) kappa B-binding heterodimeric complexes which potently transactivate kappa B-dependent constructs in transfection studies. It is concluded that human RelB (I-Rel) and murine RelB can both function as transactivators and that no significant species-specific differences exist.