We have obtained a hybridoma clone, JLJ5a, which secretes monospecific antibody directed against a 110-kdalton protein of gerbil fibroma cells, Rat-1 fibroblasts, and L6 myoblasts. It appears to be localized in the Golgi apparatus by the following criteria: (a) In double- staining experiments the localization of the 110-kdalton protein by the JLJ5a monoclonal antibody was coincident with the reaction products of thiamine pyrophosphatase (one of the enzyme markers of the Golgi apparatus; Novikoff and Goldfischer, 1961, Proc. Natl. Acad. Sci. U.S.A. 47:802-810) in the same cells. (b) The staining pattern of the JLJ5a monoclonal antibody became fragmented and dispersed into vacuoles after pretreatment of the cells with Colcemid or monensin.