Five established cell lines of human endometrium, two of normal endometrium and three of proven tumorigenicity, have been compared in terms of morphology and chromosomal numbers. Each of the five cell lines was then analyzed using immunocytochemical techniques to show that the epithelial and stromal elements could be separately identified. Antibodies directed against cytokeratin and desmoplakins were used to identify epithelial elements and antibodies directed against fibronectin were used as a marker for stroma. These results were then confirmed using Western blot analysis. We conclude that cell lines of human endometrium in culture can be differentiated as being of epithelial or stromal origin. Cell lines derived from reportedly normal human endometrium exhibit a stromal phenotype with a normal karyotype, whereas cells of tumorigenic human endometrial cell lines exhibit an epithelial phenotype and abnormal karyologic characteristics.