A study of interferon–cell interaction has shown that interferon becomes associated with cells (uptake) and that this association is partly reversible. It was further found that "uptake" of interferon was induced by low concentrations of interferon and that this process required host cell messenger ribonucleic acid (mRNA) and protein synthesis. The results of this study are interpreted as indicating that interferon itself is the viral inhibitory substance and the protein induced by interferon is associated with interferon transport.