Multiple sclerosis (MS) patients and normal subjects were tested for cellular hypersensitivity, using the leukocyte migration inhibition test (LMI), to a parainfluenza-1-virus (6/94), previously isolated from brain biopsy tissue of an MS patient. The response of MS patients as a group was not greater than that of normals to 6/94, but a significant increase in the response was observed among selected patients in the early active and chronic advanced stages of the disease. MS patients, however, did not differ from normal subjects in their cell response to Sendai virus, a similar parainfluenza type 1 virion used for comparison.