IFN-beta reduces the number and severity of exacerbations of multiple sclerosis (MS), presumably by modifying immune regulation. We used semiquantitative polymerase chain reaction (RT-PCR) to measure mRNA levels for cytokines before and after IFN beta-1b therapy. mRNA was extracted from mononuclear cells of nine healthy controls and 31 patients with MS. Before therapy, IL-10 and leukemia inhibitory factor (LIF) mRNA levels were elevated in stable MS compared to active MS. Twenty four hours after IFN beta-1b treatment, mRNA levels for IL-1, IL-2, IL-4, IL-6, IL-10, IL-12, IL-13, IFN-gamma, TNF-alpha and LIF had not changed. At 1 week, TNF-alpha mRNA increased and IL-10 and LIF mRNA rose in 75% of patients. IL-2, IL-4, IL-12, IL-13 and IFN-gamma did not change. At 3 months, cytokine mRNA returned to baseline levels. mRNA for the IFN-induced antiviral enzyme, 2,5-OAS, rose by 24 h, peaked at 1 week, and remained elevated thereafter. Serum triglycerides and liver enzymes rose after therapy. Increased SGPT at 3 months correlated with TNF-alpha mRNA levels, suggesting that cytokines may cause some side effects of IFN beta-1b. Baseline cytokine mRNA levels reflect disease activity, but the therapeutic effect of IFN beta-1 b does not appear to be explained by changes in cytokine mRNA levels.