AbstractCells have been found to contain tissue specific intermediate filaments (7–10nm). Exact determination of cellular origin can be made by using antibodies against intermediate filaments. Epithelial cells are rich in intermediate filaments of the keratin type. Using this knowledge and a polyclonal antibody to keratin filaments, individual cells from menstrual blood flow, a mixed cellular population, can be localized and specifically identified.Cells cultured from menstrual blood flow were fixed in cold methanol and stained with anti-keratin polyclonal antibody using immunoperoxidase techniques. Positively stained cells, indicating endometrial origin, revealed a diffuse filamentous network throughout the cytoplasm. Cells rounded during trypsinization, resulting in the alteration of the pattern of keratin filaments. In addition to diffuse staining throughout the cytoplasm, a dense droplet was seen juxta nuclear. By using immunogold staining techniques, localization of the keratin filaments before and after trypsinization was studied. (The J Histotechnol12:109, 1989.)