The cytotoxic cytokines tumor necrosis factor alpha (TNF-alpha) and lymphotoxin (LT) possess toxic activity against myelin and/or oligodendrocytes in vitro. Multiple sclerosis (MS) plaques within the central nervous system (CNS) are infiltrated by peripheral blood mononuclear cells (PBMC). In this study the production of TNF-alpha and LT by PBMC in active MS were measured. PBMC were isolated from the blood of MS patients in relapse and also patients with other neurological diseases (OND) and healthy controls (HC). Isolated cells were cultured unstimulated or stimulated with phytohemagglutinin A (PHA), lipopolysaccharide (LPS) and myelin basic protein (MBP)--a hypothetical autoantigen for MS. Cytokine production was assessed using ELISA method. In the MS group, PBMC without stimulation as well as after stimulation with MBP displayed a significantly increased production of TNF-alpha. LT production was similar in MS and control groups. These results suggest that TNF-alpha but not LT is overproduced by PBMC during MS relapse.