are large numbers in the red pulp. In the vertebral bone marrow, Kurloff cells are seen scattered as single cells and in clumps in random pattern among the hematopoietic cells in the stroma. Besides, the veins and lymphatic vessels of these tissues contain a significantly large number of Kurloff cells relative to the numbers in the arterial blood supply. It has been suggested that Kurloff cells might be produced at these sites. A small amount of Kurloff cells are observed within lymph nodes, and those which are present are in sinuses and blood vessels. During pregnancy and after treatment, a large number of Kurloff cells circulated in the blood. At the placenta of a pregnant guinea pig, Kurloff cells are more concentrated at the vascular channels of the labyrinth than the plasmodial spongy zone Characteristics In guinea pig, the Kurloff cells have some characteristics of monocytes and lymphocytes. The first characteristic that Kurloff cells share with monocytes is the presence in the blood and organ but absence from the spleen white pulp, thymus parenchyma, and lymph node follicles. The second shared characteristic is that the Kurloff cell has a receptor for cytophilic immunoglobin. On the other hand, Kurloff cells have many lymphocytic features such as it does not contain lysosomes or phagosomes, lack of nonspecific esterase activity, and phagocytic capacity, and last, the Kurloff cell with the early inclusion body possesses a lymphocyte morphology. The results of antibody-dependent cellular cytotoxicity assay on guineas-pig Kurloff cells strongly indicate an immunological role of the cells as the natural killer cell. The data also suggest that the Kurloff cell is probably the predominant cytotoxic line in guinea pig lymphocyte composition, and different level of killer cell activities depends mostly on varying degrees of Kurloff cell contamination. The Kurloff cells were suggested to have an impact on the immunological status of the guinea-pig placenta. The fate of cells in pregnant guineas pig has been studied by immunofluorescent methods, including an antiserum against the isolated mucoprotein of the inclusion body. The satisfactory staining of the inclusion particle was extracted from the spleen and thymus. Immunofluorescence results showed that during pregnancy circulating Kurloff cells release their inclusion particles inside the trophoblast. In the placenta, the Kurloff cell was occasionally surrounded by a cloud of tiny droplets of material, which gives a similar staining result as the inclusion particles. That material was frequently observed displaying on the