Recently it has become evident that "second growth factor" of growth hormone (GH), such as somatomedins, has an effect on the proliferation and growth of tumor cells derived from nervous tissue. Effects of host-immunocompetence and the host-humoral states on the take incidence and proliferative activity of brain tumor cells were studied using two animal models: nude mouse and pituitary Snell dwarf mouse. Nude mouse is known to be immunodeficient. Pituitary Snell dwarf mouse is characterized by lack of circulating GH, TSH, prolactin, in addition to immunodeficiency. Cell line used in this experiment was C-6 cell of rat glioma cell. After intracranial implantation of C-6 glioma cells in the animals, the take incidence and growth rate of C-6 glioma cells were followed up and measured over a period of 2 months. Tissues of implants were studied immunohistochemically and biochemically. Regardless of cell line, successful take incidence in the different animal species was found to be greater in the descending order of nude mouse, dwarf mouse. This confirmed the role of immune status for the successful take of iso-, or heterologous tumor cells after implantation. We are now investigating the effect of exogenous GH on the growth rate of cells implanted in the dwarf mouse. This may clarify the effect of growth factors on proliferative activity of implanted tumor cells.