Free light chains in the cerebrospinal fluid of patients with multiple sclerosis correlate closely with disease activity. As part of a double-blind placebo controlled trial of immunosuppression using cyclosporin A in patients with multiple sclerosis, light chain analysis was performed on pre- and post-treatment CSF samples in 19 patients, nine having received cyclosporin A and 10 placebo. The placebo treated patients showed continued immunological activity, as evidenced by an increase in both free kappa and lambda chains, but cyclosporin A treatment resulted in no change or an improvement in light chain response, indicative of suppression of continued activity. As cyclosporin A does not enter the CSF, this implies that systemic stimulation is at least partly responsible for continued disease activity in multiple sclerosis. Cyclosporin A may thus be effective in preventing the deterioration of patients with multiple sclerosis.