We have identified a novel splicing factor, Isy1p, through two-hybrid screens for interacting proteins involved in nuclear pre-mRNA splicing. Isy1p was tagged and demonstrated to be part of the splicing machinery, associated with spliceosomes throughout the splicing reactions. At least a portion of the Isy1 protein population is associated with snRNAs; low levels of U5 and U6 snRNAs are coimmunoprecipitated specifically with Isy1p. When the ISY1 gene was knocked out, no defect in vegetative growth was observed. Using a sensitive in vivo splicing assay, however, we observed lower splicing efficiency in the isy1 null mutant compared to wild-type, indicating that Isy1p is important in the optimization of splicing.