Summary. The use of high voltage paper electrophoresis for studies on the breakdown of amino acids by bacteria is described. Examination of a number of different isolates from the alimentary tract of the pig showed that the decarboxylase activity was restricted to Escherichia coli and one strain of Lactobacillus fermenti. In some isolates studied the optimum pH of activity differed from those previously reported for similar systems, being higher for ornithine, glutamic acid and lysine decarboxylases. The heterofermentative lactobacilli all converted arginine to ornithine and this may contribute to the final level of putrescine in the gut by providing a substrate for the ornithine decarboxylase of E. coli.