GroEx, a heat shock gene analogous to groE of E. coli, cloned from the symbiotic X-bacteria in Amoea proteus contained two open reading frames encoding GroESx and GroELx. When groEx was cloned in E. coli, the GroESx and GroELx were over 50% of total proteins. Although, unlike those of E coli, GroELx was largely expressed at all temperature by its unique and strong promoters, the level of GroESx was controlled by heat shock consensus promoter and was increased by heat shock. The GroES and GroEL of E coli represent major molecular chaperones that participate in folding and assembly of a variety of proteins. They are also highly expressed in pathogenic bacteria with unknown chaperonin function. Compared with chaperonin function of GroEL (hsp60), that of GroES (hsp10) is less well documented due to the lack of functional markers. In this study we produced mAbs against the GroESx and used them in identifying the expression of GroESx from X-bacteria in symbiosis. Western blotting analysis of cloned E. coli and of E. coli itself showed a single protein band of 10 kDa, which was increased by heat shock. Immunofluorescence studies on the symbiotic xD strain of A. proteus revealed a strong positive signal in symbiotic vacuoles, the nucleus and some weak signal in the cytosol. Besides, a weak signal was also shown in tD Amoeba specially in nucleus and some in the cytosol. Thus, the mAb appeared to be a useful marker to monitor the expression and detect hsp10 analogues from cloned E. coli and other cells.