We report characteristics of the cerebrospinal fluid (CSF) pleocytosis (616+/-148 cells/microliter) that occurred in guinea-pigs with definite clinical experimental allergic encephalomyelitis developing 12 to 16 days after sensitization with homologous myelin basic protein. This pleocytosis was not present in the cerebrospinal fluid of a group of animals studied when still healthy, 9 or 10 days after similar sensitization. Eighty-nine per cent of cells in the CSF pleocytosis were small lymphocytes, 8% were larger lymphocytes and the remainder mostly monocytes. Of the lymphocytes, most were E-rosetting or null cells. B-cell markers were uncommon. The cellular patterns in this CSF pleocytosis appear to be similar to those seen in some delayed hypersensitivity responses.