We have generated a bank of temperature-sensitive (ts) Schizosaccharomyces pombe mutant strains. About 150 of these mutants were transformed with a ura4 gene containing an artificial intron. We screened these ts mutants for mutants deficient in splicing of the ura4 intron. With this approach three mutants were isolated which have a general defect in the splicing process. Two of these mutants fall into the prp1 complementation group and one defines a new complementation group, prp4.