Lymphocyte subpopulations were determined in cerebrospinal fluid (CSF) from 11 patients with acute aseptic meningitis. The patients had a significantly higher percentage of T lymphocytes (as determined by rosette assays for sheep erythrocyte (E) receptor-bearing cells) in their CSF (86%) than in blood (62%). The percentage of complement (C) receptor-bearing cells was also higher in the CSF (47%), and the sum of E and C receptor positive cells was 133%, indicating a considerable number of cells carrying both receptors in the CSF in contrast to in peripheral blood. However, 8 patients with various neurological diseases had a mean of 65% C receptor-bearing cells in their CSF, while 17 others had 78 +/- 9% E receptor positive cells. Meningitis patients had a lower percentage of CSF Fc receptor-bearing cells (EA-RFC) than other patients. These results indicate the presence of a large percentage of CSF lymphocytes with both E and C receptors also in patients without meningeal disease, and suggest the recruitment of such cells into the CSF.