THE estimation of l(+)-lysine, l()-histidine, l(+)-ornithine, l(-)-tyrosine and Z(+)-glutamic acid in protein hydrolysates by the use of specific preparations of the corresponding amino-acid decarboxylases has been described previously1. A coliform organism has now been isolated which is specific for the decarboxylation of l(+)-arginine and has been deposited in the National Collection of Type Cultures under the reference number 7020. The organism is grown for 20–24 hr. at 25° C. in tryptic digest of casein containing 2 percent glucose and the crop then centrifuged down and washed once in distilled water. The washed organism is made up into a thick cream in water, poured into 5 vol. of ice-cold acetone and stirred vigorously until coagulation occurs. The coagulum is filtered off on a Buchner funnel, washed once with acetone and once with ether, dried under suction, powdered and finally dried overnight in vacuo. The acetone-powder so prepared can be used for the estimation of arginine without further purification and the dry powder retains its arginine decarboxylase activity for some weeks.