The cells, which were cultivated from the omentum of BAS-sensitized rabbits and grown in our special culture medium, contained antibodies to BSA, as detected by immunohistochemical and immunochemical methods. When the antigen (BSA) was introduced into the culture, the cells showed momentarily characteristic morphologic changes suggesting a decreased activity of cell membrane. The antibodies in the cells seemed to be due to 19S antibodies, because the morphologic changes in the cells, passively sensitized with 19S antibodies, were undoubtedly indistinguishable from those in the above actively sensitized cells. On the other hand, the cells, passively sensitized with 7S antibodies, showed momentarily different morphologic changes suggesting an increased activity of the cell membrane. These observations possibly indicate an important relationship between the chemical structure and biologic action of these two antibodies to BSA. At present, we call the cells ‘omentum cell’; and they may by very useful in the study of the formation and function of antibodies.