Ce1S is the most abundant subunit and an exoglucanase component of the Clostridium thermocellum cellulosome, multicomponent cellulase complex. The product inhibition pattern of Ce1S was examined using purified recombinant Ce1S (rCe1S) produced in Escherichia coli. The rCe1S activity on cellopentaose was strongly inhibited by cellobiose. The rCe1S activity was also inhibited by lactose. Glucose was only marginally inhibitory. Cellobiose appeared to inhibit the rCe1S 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 Ce1S is a rate-limiting factor in the activity of the cellulosome.