Cellular hypersensitivity to myelin basic (A1) protein was evaluated using the MIF assay in 246 subjects. Of 100 with multiple sclerosis positive results are seen in relationship to acute exacerbations of illness. Normal control subjects gave a mean value of 100 +/- 9% whereas patients studied within 4 weeks of onset of illness gave a result of 59 +/- 12.5%. A convalescent group studied between 5 and 12 weeks after an attack gave results of 86 +/- 22.2%. A chronic group gave a mean of 91 +/- 8.2%. Positive values were also seen in a number of other patients with central or peripheral nervous system disease especially those with "autoimmune disease". However, results of this study clearly establish a temporal relationship between in vitro evidence of hypersensitivity to A1 protein and clinical expression of disease.