Since an earlier communication1on the cultivation ofBacillus lepræand the experimental production of leprosy by one of us (Duval) considerable light has been thrown on further study into the biology of the organism, especially with regard to its pathogenicity and viability, properties little understood prior to the cultivation of the bacillus. In the present paper we propose to give briefly our researches in this particular phase of the subject together with the results of our studies in immunity. Soon after the cultivation of the specific organism the possibility of a serum therapy for the treatment of leprosy presented itself, and with this in view we set about to study the blood of lepers in the hope that something might accrue which would aid in subsequent work on the artificial production of an immune serum. Before beginning the work along this line it was thought advisable