[Isolation of the Fab fragment from immune sera].

A Fab-fragment was obtained from rabbit anti-swine JgG serum through the breakdown of JgG by means of the proteolytic enzyme pepsin in ion-exchange chromatography on DEAE cellulose. It was found that the Fab-fragment gave no positive precipitation in the immunodiffusion agar gel test, and in electrophoresis on 10 per cent acrylamide gel following preliminary breakdown with 1 per cent SDS and 2 per cent ME and heat treatment at 100 degrees C for 2 minutes it showed only one polypeptid chain with equal mobility as that of the light JgG chain. On the basis of the results obtained speculations are made of the mode and site where pepsin supposedly acts.