Humoral and cellular immune reactions against Mycoplasma pneumoniae (MPn) were investigated in 18 multiple sclerosis (MS) patients. All patients were in the remission stage. Complement-fixing antibodies against MPn were present in serum and cerebrospinal fluid, concentrated to contain the same protein levels. The CSF titres after concentration were as high as or higher than the corresponding serum titres, thus indicating intrathecal antibody production. Sensitization to MPn was demonstrable in all 18 MS patients by the antigen-reactive active E-rosette assay and antibody-dependent cellular cytotoxicity assay, in 17 of the 18 patients by the lymphocyte transformation test and in 8 patients by the cell-mediated cytotoxicity assay. The possibility of a pathogenetic role of MPn for MS is discussed.