In order to reproduce some of the pathological features of multiple sclerosis (MS) we transplanted peripheral blood lymphocytes (PBLs) from seven patients with MS into the peritoneal cavity of 28 severe combined immunodeficiency (MS-SCID) mice. Seven SCID mice were also transplanted with PBLs from two healthy subjects (hu-SCID). Animals were sacrificed between 2 and 8 weeks after transplantation (a.t.). Polymerase chain reaction (PCR) using primers able to amplify the HLA-DQ alpha region showed presence of human cells in neural tissues of MS-SCID mice. Immunocytochemical analysis revealed the scattered appearance of human lymphocytes (mostly CD45RO+ T cells) in the meningeal space and choroid plexuses of MS-SCID brains. However, human lymphocytes were similarly found in brains of hu-SCID mice. Both groups of mice never showed signs or symptoms of neurological impairment. Our results indicate that the simple transplantation of lymphocytes from MS patients into SCID mice is not likely to produce an MS-like pathology.