A potent nonspecific inhibitor of hemagglutination by DA virus exists in the allantoic and amniotic fluids of normal chick embryos. The inhibitor concentration rises with advancing age of the embryo and reaches a peak concentration on the 18th day. Egg-adapted virus appeared to be more sensitive to the inhibitor than did virus cultivated in monkey kidney tissue and it was shown that this difference might be due to changes in the surface configuration of the virus following adaptation to the chick embryo, which resulted in a possible “steric effect.” Unlike the inhibitor of PR8 virus previously described for this system, the DA virus inhibitor seemed to be a muco-polysaccharide. The inhibitor was destroyed during active virus multiplication and possessed no neutralizing effect on infectivity of the virus.

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