The inflammatory nature of multiple sclerosis (MS) implicates the participation of immunoregulatory cytokines, including the Th2 related IL-10. We describe the use of in situ hybridization with cDNA oligonucleotide probes to detect and enumerate mononuclear cells (MNC) expressing mRNA for IL-10, which is known to down-regulate Th1 cell related cytokines such as interferon-gamma. Expression of IL-10 was studied in blood MNC of MS and blood and cerebrospinal fluid (CSF) MNC of optic neuritis (ON) patients without culture and after culture in the presence of myelin basic protein (MBP), the control antigen acetylcholine receptor (AChR), and without antigen. Numbers of IL-10 mRNA expressing MNC were elevated in the MS patients' blood both when enumerated without culture and after culture with MBP. Control patients with myasthenia gravis had elevated numbers of AChR-reactive IL-10 mRNA expressing cells, while numbers of MBP-reactive IL-10 positive cells did not differ from numbers registered in cells without antigen. Patients with ON, in many instances representing early MS, had IL-10 positive blood MNC that were elevated to the same extent as in clinically definite MS, and further increased in the CSF. ON patients examined within 1 month after onset had lower numbers of MBP induced IL-10 mRNA expressing blood MNC compared with patients examined later suggesting that IL-10 is related to the degree of inflammation and outcome in ON.