We measured beta 2-microglobulin (beta 2-M), soluble interleukin-2 receptor (sIL-2R), and soluble CD8 (sCD8) antigen levels in paired cerebrospinal fluid (CSF) and sera from patients with subacute sclerosing panencephalitis (SSPE), multiple sclerosis (MS), and other neurological diseases (OND) using enzyme-linked immunosorbent assay. beta 2-M was significantly increased in CSF of the SSPE group compared to the MS or the OND group. Similarly, beta 2-M in the MS versus OND group was significantly increased in CSF. Although serum levels of beta 2-M were similar in the three groups, the CSF/serum ratios were higher in SSPE versus the MS group and in the MS versus the OND group. Levels of sIL-2R and sCD8 were higher in SSPE CSF than OND CSF; however, there were no differences between levels in SSPE and MS CSF. The levels of sIL-2R were increased in SSPE sera compared to those of MS or the OND group, whereas levels of sCD8 in serum from the three groups were similar. The findings of increased CSF/serum ratio of beta 2-M and higher levels of serum sIL-2R and CSF sCD8 in SSPE patients are consistent with those seen in patients with acute and chronic viral infections. When the levels between the initial and follow-up CSF and serum samples from SSPE patients were compared, the data showed that CSF levels of sCD8 elevated during periods of clinical worsening and decreased during clinical improvement. In contrast, serum beta 2-M decreased during periods of worsening and increased during improvement. The measurement of serum beta 2-M and CSF sCD8 may be useful in SSPE patients as markers to monitor disease activity.