Mice adoptively‐sensitized to develop chronic relapsing experimental autoimmune encephalomyelitis (EAE), a model for the human demyelinating condition, multiple sclerosis (MS), were given injections of recombinant human IL‐10 at various timepoints post‐sensitization in an attempt to abrogate disease development. IL‐10 is a Th2 immunomodulatory cytokine with known down‐regulatory effects upon Th1 responses and macrophages. Contrary to a previous report on EAE and the predicted outcome, after repeated experiments, IL‐10 was found to elicit a worsening or no effect upon EAE in the mouse. Animals were studied clinically, histopathologically and immunocytochemically. On no occasion was disease ameliorated by IL‐10. Pretreatment with IL‐10 of lymph node cells used to transfer EAE had no effect upon disease outcome, indicating that the cells were already committed effectors. Administration of anti‐IL‐10 monoclonal antibody before onset of signs had no effect when given early post‐sensitization and caused marked worsening when given immediately before onset of signs. In the context of this autoimmune demyelinating model, these results suggest that IL‐10 alone is insufficient to reverse the effector response and indeed may serve to enhance the cascade of events in EAE. © 1996 Wiley‐Liss, Inc.