The moral justification for substitutive sterilization presupposes a critical verification of the sterilization wishes and an exact explanation to both partners. With the acquisition of the proper consent, the explanation of the absolute necessity of the postoperative spermiogram check-up cannot be neglected. Comparison of the spermiograms of unalterably vasectomized men with findings from additional rinsings with physiological saline solution and nitrofurantoin showed that the instillation of the vas deferens leads to a swift, mechanically dependent, emptying of the distal sperm depot. Through instillation of the spermicidal substance nitrofurantoin and the resulting spermatozoa immobilization, this effect can be considerably increased.