Usually investigated in men from involuntarily childless marriages. The most suitable sperm for such tests is that produced by masturbation; it must be examined under the microscope within an hour. Semen consists of fluid from the prostate gland and seminal vesicles, mixed with spermatozoa from the testicles. On ejaculation prostate fluid is expelled first, then spermatozoa, and then fluid from the seminal vesicles. Various abnormalities can occur: too few spermatozoa per ejaculation, no spermatozoa at all, too few active spermatozoa, or only dead or abnor- mally formed sprematozoa. Fluid from the prostate or seminal vesicles may be inadequate in quantity or of abnormal chemical composition. Such conditions may be congenital, as in Klinefelter’s syndrome or in the case of undescended testicles. The testicles can also be damaged by infections such as mumps, notoriously. Other conditions which can cause sperm deficiencies are bilateral testicular cysts, variocele, torsion of the testicle, lead poisoning, malnutrition and severe vitamin A and E deficiency. Such abnormalities can often not be treated, and result in sterility.