This chapter discusses the sites of genitourinary tract infections, infectious organisms, the numerous ways in which leukocytes may impair male reproduction, and the diagnosis and treatment of leukocytospermia and bacteriospermia. Infections of the urethra are most common due to sexually transmitted pathogens, and they are separated into two broad categories: gonococcal urethritis (due to Neisseria gonorrhoeae ) and nongonococcal urethritis (Chlamydia trachomatis, Mycoplasma species, Trichomonas vaginalis). Immunohistology employs monoclonal antibodies (mAb) targeted against white blood cell (WBC) surface markers. Flow cytometry, when used in conjunction with monoclonal antibodies, can provide rapid analysis of scant WBC subpopulations without purification procedures. Leukocytes, monocytes, and granulocytes are differentiated by using the light-scatter properties of the WBCs and the density of the leukocyte marker antibody, CD45. The chapter suggests that abnormal concentrations of leukocytes are a common finding in the semen of men with spinal cord injury (SCI).