When the sperm and egg interact in the body, they do so in a very complex environment made up of many biological and cellular components, some known, most not. If, because of the presence of some indeterminate substance the fertilization process is interfered with, conception will not occur. Any number of biological substances produced by the body, proteins, antibodies, or other complex molecules) could cause fertilization failure. There is no possible way to test an individual for the presence of such substances. Yet in a couple having unexplained infertility, the fact that fertilization occurs In Vitro is a very positive event. It rules out most egg problems and problems of fertilization. It also demonstrates biologically normal sperm function. By process of elimination, the block to conception could have been either due to a tubal problem or to the existence of a blocking factor In Vivo, or even both. The actual cause of infertility cannot be determined, but it can certainly be bypassed.