(b) Kinetics of induction in rabbits: The kinetics for interferon induction by I: C are shown in Figure 1. A significant level of interferon appeared by one hour, reached a peak shortly thereafter, and declinedsignificantly by four to six hours.(2) Characterization of interferon induced in rabbits by complexed polynucleotides: The viral inhibitory substances in the sera of rabbits injected with the active complexed polynucleotides were identified as interferon based on their biological and biochemical properties.(a) Host species-specificity: The rabbit serum interferon noted in Table 2 was prepared as previously described.'Mouse interferon was prepared from groups of 25 mice each, 16-18 gm in weight, injected intravenously with 10.5 jg of I: C in 0.2-ml doses. The mice were bled in two hours, the sera pooled, sterilized by ultraviolet irradiation, and titrated by the plaque reduction technique against VSV challenge. Where the presence of residual inducer in serum samples might interfere with the species-specificity determination, the interferon was purifiedby chromatography on CM-Sephadex. 6 It is seen in Table 2 that resistance was evoked in the homologous cell systems by the corresponding interferons but not in heterologous cell culture systems.(b) Trypsin sensitivity: Table 3 shows that the activity of the rabbit serum interferon was destroyed by trypsin.(c) Molecularweight: Themolecular weight of rabbit serum interferon induced by I: C was determined bygel filtration through Sephadex G-200 and was found to rangefrom 49,000 to 52,000.1 (d) Isoelectric point: The isoelectric point of rabbit serum interferon induced by I: C was measured by chromatography on CM-Sephadex and was found to be pH