Proteins associated with starch synthase (SS) activities were identified in immature mungbeans (Vigna radiata L. cv KPS1). Seed soluble extract was separated by native-PAGE and subjected to in situ activity staining. The gel zymogram located starch-enzyme complex bands. The soluble extract was also partitioned by preparative-IEF and screened for SS activity using radioactive assay. IEF fractions eluted within pH 4-6 revealed enriched SS activity of 145-fold. Parallel comparison of the protein profiles among the activity stained enzyme complex and the active isoelectric focused fractions on SDS-PAGE depicted three SS-activity-related proteins with molecular size of 32, 53, and 85 kDa. The 85 kDa protein, however, was identified to be methionine synthase by MALDI-TOF analysis and should be a protein physically associated with the active SS. Polyclonal antibodies raised from eluted native enzyme complex neutralized up to 90% activity and antigenically recognize the other 53 and 32 kDa proteins on Western blot. Antibodies raised from the two individual denatured proteins were able to neutralize SS activities near 60% separately, indicating that the 53 kDa and 32 proteins associated with SS activity are potentially involved in starch biosynthesis during mungbean seed development.