Specific cell-mediated immunity to polio antigens in 22 children suffering from acute paralytic poliomyelitis was investigated by the macrophage migration inhibition factor (MIF) technique. A positive response was observed in 9 of the 11 patients tested 1-4 days after the onset of the paralysis, and in 6 of the 8 patients tested between 15-90 days. In one case, the response was positive before paralysis became evident. In the 8 patients tested 90-360 days after paralysis, only three gave positive results. No response was observed in 34 of 39 healthy children tested 1-3 months after having received oral polio vaccine, or in 30 healthy adults.