SAP155 is a subunit of the U2 snRNP, and plays an important role for prespliceosome assembly and splicing catalysis of the major spliceosome. Recently, it was reported that SAP155 was also a subunit of the minor spliceosome. These suggest that SAP155 is essential for the removal of any type of intron. More recently, a homolog of SAP155 cDNA, designated Sf3b1, was isolated from mouse. In this study, we report the generation of a monoclonal antibody (MAb) against murine Sf3b1 protein. This MAb recognizes endogenous Sf3b1 gene product by Western blotting, but less efficiently by immunoprecipitation.