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SUMMARY: In vesicular stomatitis virus inocula containing the transmissible interfering component (T) an exponential relation, at low doses, between inoculum concentration and virus yield suggested that adsorption of one T particle was enough to exclude infective virus. This relation was not maintained at high doses of inoculum, possibly because maximal interference required several hours between adsorption of T and virus particles. An assay method for T, based on the dose which gave 37% of the yield from T-free inocula, showed that the T content of inocula was usually related to passage history and yield of virus on undiluted passage. Treatment with immune serum did not appear to neutralize T, which sedimented in the centrifuge more slowly than did virus. T was much less rapidly inactivated at 56° and by u.v. irradiation than was the infectivity. Despite resemblances between the agent T, incomplete influenza virus and interferon, direct evidence is lacking that T is an incomplete form of the vesicular stomatitis virus or an interferon-like substance.