Conversion of pre-mRNAs into mature mRNAs includes several consecutive enzymatic modification steps that are carried out in the spliceosomes. Helicases have been shown to contribute to these catalytic processes both in yeast and in mammalian cells. Our results identify the mammalian protein MAH (matrix-associated helicase) as a new helicase present in the spliceosome complex. Sequence comparison describes MAH as the first higher eukaryotic member of the helicase superfamily I, with demonstrated enzymatic activity. Because MAH does not bind small nuclear ribonucleoproteins (snRNPs), it appears to be a non-snRNP binding factor of the splicing complex. In conclusion, our data suggest the involvement of MAH in processing of pre-mRNAs in mammalian cells.