In experimental infections with a virulent strain of Trichomonas gallinae healthy flagellates were found within macrophages in mice around subcutaneous abscesses and in chick liver cell cultures. Parasites of this strain were observed also within the cytoplasm of epithelial cells in chick liver cultures (in such cultures the fibroblast-like cells were invaded also) and in sections of liver from a pigeon. Histopathologic examination of lesions produced in mice by subcutaneous inoculations of a virulent strain of Trichomonas vaginalis revealed the parasites within macrophages. In natural infections also the host phagocytes were found to contain apparently healthy trichomonads. In one instance T vaginalis was observed within human epithelial cells in a tissue biopsy of the cervix uteri.