CelS is the most abundant subunit and an exoglucanase component of the Clostridium thermocellum cellulosome, multicomponent cellulase complex. The product inhibition pattern of CelS was examined using purified recombinant CelS (rCelS) produced in Escherichia coli. The rCelS activity on cellopentaose was strongly inhibited by cellobiose. The rCelS activity was also inhibited by lactose. Glucose was only marginally inhibitory. Cellobiose appeared to inhibit the rCelS activity through a competitive mechanism. The inhibition was relieved when beta-glucosidase was added, presumably because of the conversion of cellobiose into glucose. These hydrolysis product inhibition patterns are consistent with those of the crude enzyme (cellulosome), suggesting that CelS is a rate-limiting factor in the activity of the cellulosome.