The purposes of the present work have been: (1) to summarize the work so far done on the subject, (2) to review briefly the known facts throwing light on the anatomy and physiology of the male genital organs, (3) to carry our systematic studies upon the pathology and bacteriology of the genital tract of the bull, and (4) to ascertain if possible whether the bull is a disseminator of those infections which interfere with reproduction in the female. The work has been carried on for the most part from the point of view of a laboratory man cooperating with clinicians. No attempt is made in this paper to give detailed clinical data, methods for physical examinations, etc. There are included many statements and some data, given in a preliminary article on the subject. While the subject is broad in its scope, in fact too broad for great detail, it is hoped that a start has been made toward future and more detailed investigations. HISTORY References to, and investigations relating to, the part played by the bull in the process of reproduction in the herd, and in the spread of genital infections, have been limited largely to those phenomena caused by _Bact. abortum_. Bang (1) originally called attention to the possibility of the male transmitting the organism discovered by him, but he reached no definite conclusion on the subject. James Law (2) writing on contagious abortion in cows, early suspected this possibility when stating under “casual infections,” that—“In a case which came under the observation of the writer recently, a family cow, kept in a barn where no abortion had previously occurred, was taken for service to a bull in a herd where abortion was prevailing, and though she was only present at the latter place for a few minutes, she aborted in the sixth month.” Jansen, as quoted by Sand, reports the case of a cow from an aborting herd having been taken into a herd that had been previously quite free from the disease. Soon after her arrival she aborted, and later cow after cow of the original herd aborted. The owner kept the matter a secret, and sent his cow to a neighbor’s bull for service, with the result that for two years abortion prevailed among cows served by this bull. McFadyean and Stockman (3) later, in experimental work, attempted but failed to infect cows by using a soiled bull for service. Hadley and Lothe (4) state: “A large number of stockmen hold that the bull is an important factor in the transmission of contagious abortion in herds. A smaller number believe that the bull merely acts as a passive carrier of the abortion disease and is not actively concerned in the transmission.” In a subsequent bulletin, Hadley (5) remarks: “The abortion organisms may enter the body ... during sexual intercourse.” In an experiment carried on by the same author and co-workers, abortion-free virgin heifers were mated to abortion-infected bulls, infection being evidenced