Introduction The failure of the brain to respond with a typical immune reaction when challenged by intracerebral implantation of nonisologous tissue has intrigued investigators and led to considerable speculation. Medawar1showed that the brain does not respond with the typical "primary or secondary set reaction" histologically when homologous skin is implanted in the cerebrum. Greene2-4demonstrated that the brain will accept tumor homotransplants or heterotransplants. These phenomena and the failure to demonstrate lymphatic channels in the brain have led to the concept that the brain is "an immunologically privileged site." The purpose of this report primarily is to present results of some investigations that further examine this concept. However, knowledge of the histologic events which occur when a tumor homotransplant or isotransplant is placed in the brain of the experimental animal may also prove useful in the understanding of the mechanisms by which the human brain reacts to