Two bifunctional reagents designed to probe the active site of pepsin and other acid proteinases are described. One of these, the bisdiazoketone 1,1-bis(diazoacetyl)-2-phenylethane inactivates pepsin at pH 5.0 much more rapidly than the corresponding monodiazoketon 1-diazoacetyl-2-phenylethane, whereas the other, the bromodiazoketone dl-1-diazoacetyl-1-bromo-2-phenylethane is less effective in this regard. The inactivation is greatly accelerated by the presence of Cu(II), and the pH dependence of the process is consistent with the interaction of the enzyme with the metal complex of the carbene derived from the reagent. The bisdiazoketone appears to react stoichiometrically with pepsin in a 1:1 ratio to form a product whose apparent molecular size is the same as that of untreated pepsin. The inactivation of pepsin by the bromodiazoketone is accompanied by the release of stoichiometric amounts of bromide ions and the formation of a major product whose apparent size is similar to that of pepsin, and a minor component larger than the untreated enzyme.