Lanthanide complexes covalently attached to oligonucleotides have been shown to cleave RNA in a sequence-specific manner. Efficient cleavage, however, is at present limited to single-stranded RNA regions, as RNA in a duplex is considerably more resistant to strand scission. To overcome this limitation, we have designed and synthesised artificial nucleases comprising lanthanide complexes covalently linked to oligodeoxyribonucleotides which cleave a partially complementary RNA at a bulged site, in the duplex region. Strand scission occurs at or near the bulge. Cleavage of the RNA target by the metal complex can be addressed via the major or the minor groove. In an example of a competitive situation, where the cleavage moiety has access to both a bulge and a single-strand region, transesterification at the bulge is favoured. Such artificial ribonucleases may find application as antisense agents and as tools in molecular biology. In addition, the results may have importance for the design of artificial ribonucleases which are able to act with catalytic turnover.