The activation of poliomyelitis virus from a "neutral" virus-antibody mixture by the addition of non-infectious virus is described. The phenomenon was demonstrated in tissue cultures using both Petri dishes and roller tubes. The reaction was found to be type-specific and a relationship between antigen dilution and number of plaques counted was established. The relative potency of vaccine preparations determined in this system was found to be comparable to their relative potency in respect to protection-inducing activity in mice.