The authors report two new cases of encephalitis occurring in association with immunity deficiency. The first concerns a child suffering from Bruton type congenital hypogammaglobulinemia. Histological examination of the brain under light microscopy shows important lesions of panencephalitis with parenchymal atrophy : the latter was more marked in the cerebellar lamellae. Ultra-structural examination showed numerous nuclear bodies in the nuclei of the macroglial cells and tubular inclusions situated in the cytoplasm of the capillary endothelial cells. The second case is that of a man of 44 years of age who had undergone a kidney transplant for malignant arterial hypertension, who after 3 months immunodepressive treatment suffered from temporo-spatial confusion together with a pyramidal tract syndrome. He died after 3 weeks evolution of the disease. Microscopical examination of the encephalon showed the presence of microglial nodules with aspects of neurophagia, suggestive of a polioencephalitis. These lesions were situated in the temporal cortex and the brain stem. The lymphocytic reaction was very slight in the second case.