The processing of a 57-kDa peptide into 37- and 22-kDa subunits of glutelin, a major storage protein of rice, was confirmed by the immunological compatibility between the precursor and glutelin subunits. The 57-kDa peptide reacted with the antisera raised against purified 37-kDa and 22-kDa subunits of glutelin. The processing was further confirmed by alteration of an in vivo protein synthesis by monensin, a sodium ionophore which inhibits the intracellular transport of secretory and membrane proteins. Infusion of monensin into developing rice grains resulted in suppressed formation of mature glutelin subunits with concomitant accumulation of the 57-kDa peptide. The present results indicate that both subunits of rice glutelin were produced by post-translational cleavage of the 57-kDa peptide.