THE FAILURE of the brain to respond with a typical immune reaction when chailenged by intracerebral implantation of nonisologous tissue has led investigators to the concept of the brain as an "immunologically privileged site."1-4 We have previously investigated this concept by using intracerebral tumor homografts and intracerebral tumor isografts to sensitized animals and concluded that probably due to variable genetic relations and other factors not yet completely understood, the brain is only partially "immunologically privileged" in tumor transplantation experiments in various strains of inbred mice.5 In that study, using a transplantable glioma arising in C57BL/6J mice, it was shown that this tumor was rejected when transplanted to the subcutaneous tissues of six other strains of inbred mice. When transplanted to the brains of these same strains of mice, the tumor grew in 10 to 35% of those mouse strains, although it had been rejected in 100% of