The serum levels of the heterodimeric cytokine IL-12 were measured by solid-phase ELISA in a group of healthy subjects, multiple sclerosis (MS) patients with secondary chronic progressive course of the disease and patients suffering from other neurological diseases (OND). Serum levels of IL-12 higher than 5 pg/ml (limit of sensitivity of the assay) were only found in 2/30 (6.7%) of the healthy subjects and none of the 8 subjects with OND. In contrast, IL-12 was found in the majority of CPMS patients' sera (10/15, 66.7%) with values ranging between 5.5 and 18.6 pg/ml. These results are suggestive for an up-regulated production of IL-12 in CPMS.