NOTCH belongs to a family of transmembrane proteins that are widely conserved from flies to vertebrates and are thought to be involved in cell-fate decisions. In Drosophila, the Suppressor of hairless (Su(H)) gene(1,2) and genes of the Enhancer of split (E(Spl)) complex, which encode proteins of the basic helix-loop-helix type(3,4) have been implicated in the Notch signalling pathway. Mammalian homologues of E(Spl), such as the mouse Hairy enhancer of split (HES-1)(5), have been isolated. Both HES-1 and the intracellular domain of murine Notch (mNotch) are able to block MyoD-induced myogenesis(5-7). Here we show that activated forms of mNotch associate with the human analogue of Su(H), KBF2/RBP-J kappa (refs 8,9) and act as transcriptional activators through the KBF2-binding sites of the HES-1 promoter.