In this paper we describe experiments to determine whether dialysable extracts of non-lymphoid and lymphoid organs augment lymphocyte transformation in vitro in a manner similar to the augmenting activity of leucocyte dialysates. Human peripheral blood leucocytes were cultured with tuberculin PPD or leucoagglutinin, and dose-related effects of the dialysable extracts on lymphocyte transformation were studied by 125IUdR incorporation. Augmentation of lymphocyte transformation was obtained not only with leucocyte dialysates but also with dialysable extracts of lymphoid and non-lymphoid organs (e.g. spleen, liver, kidney, brain). It is concluded that the agent or agents present in dialysable leucocyte transfer factor preparations, which augment lymphoid transformation in vitro, are widely distributed throughout mammalian tissues.