Cellular elements from the mucous membrane of the uterus and oviducts and from peritoneal washings were cultured. The in vitro behavior of these cells was compared to elucidate the histogenesis of endometriosis and the role of various diagnostic procedures. In 65% of the cultured material obtained by uterine-tubal flushing, proliferating cells of the uterine-tubal mucous membrane were present. Their morphology and behavior corresponded to those of cultured cells obtained by separate washing of the uterine cavity and the tubes, respectively, curetted material, and biopsies of endometriosis lesions. Epithelial and stromal cells were identified using phase contrast microscopy, electron microscopy, and immunohistochemical methods. These cell types did not occur in peritoneal washings before the flushing of uterus and tubes. It was therefore assumed that they were detached and transported to the pelvic cavity during the above-mentioned procedures. In view of their intensive proliferation they may form the basis in the development of nodules of endometriosis. This would support the implantation theory concerning the pathogenesis of endometriosis. Interactions between epithelial and mesothelial cells point to the possible role of the latter in encapsulating the endometrial elements.