To successfully dissect molecular pathwaysin vivo, there is often a need to use specific inhibitors. Bacitracin is very widely used as an inhibitor of pro-tein disulfide isomerase (PDI) in vivo. However, the specificity of action of an inhibitor for a protein-folding catalyst cannot be determined in vivo. Furthermore,in vitro evidence for the specificity of bacitracin for PDI is scarce, and the mechanism of inhibition is unknown.