Bovine sera capable of inhibiting plaque formation of poliovirus may be highly selective in their action. A bovine serum that strongly inhibited plaque formation of the CHAT type 1 attenuated poliovirus had little effect on the 24 other type 1 strains tested, whether attenuated or virulent. An inhibitory factor, located in the 19 S serum globulin fraction by gel filtration with Sephadex G-200, did not affect replication of the sensitive virus, but it did initially neutralize virus in a cell-free system according to first order kinetics. The neutralized virus was reactivated at pH's below 5.0.

In correlating the inhibition in plaque reduction tests with the effects of cell-free neutralization, three results were observed: a) virus sensitive in both systems, b) virus nonsensitive in plaque reduction tests but sensitive to direct incubation with inhibitory serum and c) virus relatively nonsensitive in both systems.

In experiments with sequential neutralization, the addition of fresh bovine serum to mixtures containing virus previously exposed to bovine serum did not cause neutralization of the remaining infectious virus. However, type 1 antiserum did neutralize additional virus, although the amount neutralized and the rate of the second reaction were both somewhat diminished. Conversely, bovine serum added to complexes of virus and type 1 antiserum did not neutralize the residual virus.

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