Intranasal administration of 10(4) U of murine interferon (IFN)-alpha/beta prolonged the survival time of DBA/2 mice injected i.v. with 10(5) (> 20,000 LD50) IFN-alpha/beta-resistant 3C18 Friend leukemia cells (FLC). Long-term survivors rejected a second challenge with 3C18 FLC without additional IFN-alpha/beta treatment. IFN-alpha/beta administered intranasally was not effective in 3C18 FLC-challenged DBA/2 mice pretreated with antibody to IFN-alpha/beta. Recombinant human IFN-alpha BDBB, which is cross-reactive on mouse cells, also increased the survival time of 3C18 FLC-challenged DBA/2 mice. Placing 10(4) U IFN-alpha/beta twice a day into the nostrils also prolonged the survival time of DBA/2 mice with 3-day established 3C18 FLC tumors. Administration of 10(4) U IFN-alpha/beta into the nasopharynx was equally effective or more effective than an equivalent amount of IFN-alpha/beta given by the i.p or oral routes or by gavage. Intranasally administered IFN-alpha/beta also increased the survival time of C57B1/6 mice challenged i.v. with EL4 T cell lymphoma cells, DBA/2 mice challenged i.v. with L1210R B cell lymphoma cells, and C57B1/6 (H-2b) and DBA/2 (H-2d) mice challenged i.v. with B16 melanoma cells (H-2b). These results may be important in devising novel therapeutic strategies for malignant disease using type I IFN.