Numerous experiments have now established that the life-span of the corpus luteum is regulated by the uterus in a number of species, including the pig (see review by Short, 1967). The regression of the corpus luteum during the normal oestrous cycle in the non-pregnant animal is dependent on the endometrium; the life-span of the corpus luteum can be maintained by removing the endometrium, or by transferring an embryo to the uterus. Experiments of this nature have led to the concept of a uterine luteolytic hormone and numerous workers have attempted to demonstrate the existence of such a substance in uterine extracts without success save for one study in vitro (Duncan, Bowerman, Anderson, Hearn & Melampy, 1961). The reason for these failures may have been the inadequacy of the test systems used. Channing (1966) has recently shown that granulosa cells can be grown in tissue culture after harvesting them from the