There is now evidence that major depression is accompanied by activation of the inflammatory response system (IRS) as indicated by an increased production of pro-inflammatory cytokines. There is circumstantial evidence implicating pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNFalpha) in the pathogenesis of multiple sclerosis (MS). The aims of the present study were to examine (i) the serum concentrations of interleukin-6 (IL-6), IL-8, TNFalpha, IL-2 receptor (IL-2R) and CC16 (uteroglobulin), an endogenous anti-cytokine, in depressed and MS patients compared to normal controls, and (ii) the effects of treatment with antidepressants on the above IRS variables in depressed patients. Serum TNFalpha was significantly higher in depressed and MS patients than in normal controls. Serum IL-8 was significantly higher in depressed patients than in patients with MS. Serum CC16 was significantly higher in patients with MS than in normal controls and depressed patients. Nonresponders to treatment with antidepressants had significantly higher serum IL-2R and lower serum CC16 concentrations than responders to treatment. The results show that (i) depression is accompanied by activation of the IRS and that this activation is more pronounced in depression than in MS, and (ii) IRS activation in depressed patients is related to a nonresponse to treatment with antidepressants.