A second injection of 100 mug poly (rI) poly (rC) per mouse at 6 and 24 hours after the first injection stimulated additional peaks of interferon production. The dynamics of the process of accumulation and disappearance of interferon was similar to that after a single injection of poly (rI). poly (rC). Injection of the above dose 12 hours after the first injection induced no interferon production as it apparently coincided with the refractory state in interferon production. After pretreatment of poly (rI) poly (rC) with DEAE-dextran, the refractory phase occurred in 6 hours. Inoculation of Venezuelan equine encephalomyelitis virus as a second interferon inducer resulted in a repeated stimulation of interferon production both in animals and in tissue culture; however, interferon titres in this case were low. The use of an inactivated virus as a second interferon inducer stimulated interferon production to higher titres (5120 IU/ml) than a single injection of DEAE-dextran-treated poly (rI). poly (rC). It is possible that a combined use of poly (rI). poly (rC) and noninfectious virus as a second interferon inducer eliminates the development of the refractory state.