The xylanase gene (xynB) from Clostridium stercorarium was expressed in tobacco cells (BY2) under the control of the cauliflower mosaic virus (CaMV) 3SS promoter. The gene product, xylanase B (XynB), was proteolytically processed in the transformed cells (termed B1-11) : a larger form of the protein was mainly found in the culture supernatant and a smaller one was found in the cells. The amount of XynB protein was estimated to be 5% of the total soluble proteins in B1-11 calli after 2 weeks of cultivation. XynB isolated from the transformed tobacco cells had the activity to hydrolyze the cell wall of barley straw.