The MSL5 gene, which codes for the splicing factor BBP/ScSF1, is essential in Saccharomyces cerevisiae, yet previous analyses failed to reveal a defect in assembly of (pre)-spliceosomes or in vitro splicing associated with its depletion. We generated 11 temperature-sensitive (ts) mutants and one cold-sensitive (cs) mutant in the corresponding gene and analyzed their phenotypes. While all mutants were blocked in the formation of commitment complex 2 (CC2) at non-permissive and permissive temperature, the ts mutants showed no defect in spliceosome formation and splicing in vitro. The cs mutant was defective in (pre)-spliceosome formation, but residual splicing activity could be detected. In vivo splicing of reporters carrying introns weakened by mutations in the 5' splice site and/or in the branchpoint region was affected in all mutants. Pre-mRNA leakage to the cytoplasm was strongly increased (up to 40-fold) in the mutants. A combination of ts mutants with a disruption of upf1, a gene involved in nonsense-mediated decay, resulted in a specific synthetic growth phenotype, suggesting that the essential function of SF1 in yeast could be related to the retention of pre-mRNA in the nucleus.