A portion of the seed virus remained active even in the presence of excess antibody. With an early serum or relatively low concentrations of a hyperimmune serum, the presence of such unneutralizable virus was uncovered by the addition of complement. This unneutralizable persistent fraction (PF) was distinguished from sensitized virus surviving in insufficient antibody, because the latter was inactivated by high concentration of antibody as quickly as the control virus. The level of the PF was constant regardless of the serum species, serum lot, dilution of serum, antibody class and complement requirement of antibody, being approximately) 0.1% of the seed virus. Millipore filtration of the seed virus lowered this PF level to varying degrees depending upon the porosity However, when filtered virus-serum mixtures were incubated at 37C for a Sufficient time and then filtered again, a portion of the unneutralized virus did pass a 0.22μ membrane. Furthermore, virus sensitized with insufficient antibody showed no increase in resistance to complement after 10hr incubation at 4C, These results proved that viral aggregation was not the essential cause of the PF. It was confirmed, on the other hand, that the neutralization of virus by hyperimmune IgG followed a one-hit curve. Analysis of these data appeared to support Rappaport's postulation that the PF represented antibody-coated virus whose critical antigenic sites were all left unbound.