A method is described for extracting and comparing internal granule proteins from a normal commercial soft wheat variety (QAL2000) and related wx starch mutants (QA-Wx-59 and QA-Wx-1). Our results demonstrate that granule-bound starch synthase I (GBSS I) was by far the most abundant internal protein of starch granules from QAL2000. Several minor, smaller bands observed in gels were also identified by MS as GBSS I peptides. The GBSS I peptides were detected in only trace amounts in the wx mutants. Proteins corresponding to other starch biosynthetic enzymes were present in QAL2000 and the wx mutants.