The ability of five human colonic bacteroides (B. fragilis, B. splanchnicus, B. thetaiotaomicron, B. vulgatus, and B. ovatus) to grow on pancreatic trypsin and chymotrypsin, as well as other organic nitrogen sources, was studied in batch culture. All isolates grew on trypsin and, with the exception of B. thetaiotaomicron, on chymotrypsin. The proteolytic activities of these bacteria varied considerably. However, the ability to utilize either trypsin or chymotrypsin was not related to the proteolytic potential of the bacteria and probably depended upon their ability to assimilate low-molecular-weight peptides generated during autolysis of the enzymes.