The experiments reported describe the actual cultivation of Mycobacterium leprae on artificial culture medium in a gaseous tension of carbon dioxide and oxygen through sixteen generations over a period of eighteen months. Several new mediums have been prepared and inoculated but no growth appeared after six weeks incubation. Organisms from the ninth, tenth and eleventh serial transfers failed to produce experimental tissue changes in older monkeys then were used several months earlier when the culture which was not so distant from the host produced granulomatous lesions by intradermal injection. Guinea pigs and several varieties of mice manifested no pathologic changes when injected intracutaneously with these cultures. Serological data are given as a matter of record. A discussion of the possible mechanism of infection in human leprosy is presented.