A considerable fraction of the influenza A virus contained in infected allantoic fluid of the developing chick is not sedimentable under conditions which remove virus activity almost completely from filtrates of emulsified mouse lung. The infectious unit from tissue suspensions is about 100 mµ in diameter and is of the same chemical composition as particles of the same size and abundance separated from normal tissues by an identical procedure. Evidence has been presented showing that the infectivity can be, and probably is, carried on such normal cell components as an adsorbate. Other non-infective particles such as erythrocytes may also become infectious units through adsorption of the virus.

The virus occurs in allantoic fluid in two states of dispersion. A variable percentage is associated with particles considerably less than 100 mµ in diameter, probably more nearly 10 mµ, while the remainder is reversibly aggregated. Reversal to the more disperse state may be effected by dilution, sonic vibration, or moderate heat treatment.

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