THE Commission on Enzymes of the International Union of Biochemistry1 recommends that the highly active pepsin prepared from pig gastric mucosa2 which is presumed quantitatively to be the principal pepsin should retain the name originally given to it, pepsin A; the other pig pepsins3,4, initially called parapepsins, should be named B. There are three of these minor pepsins, and so Ryle5 has subsequently used the name B for the former parapepsin I, C for parapepsin II and D for the third. But this alphabetical system is now inadequate, for B has been found to have two components5. Fortunately, none of the groups working with human material has used the A, B, C, D nomenclature, so that for the time being these initials, which are also given to the corresponding pepsinogens, can still be used for the porcine pepsins.