The immunogenicity of influenza A strain A/Northern Territory/60/68 for mice when delivered by the ocular, nasal and sub-cutaneous routes was determined according to the dose of infectious virus required to induce inhibition of multiplication of a standard intransal challenge dose in 50 per cent of animals per group 3 weeks after vaccination. For mice inoculated by the ocular route, an immunizing dose of 10^2.89 TCID₅₀ per animal was required. For anaesthetised mice vaccinated intranasally and unanaesthetised mice vaccinated sub-cutaneously these figures are <>^2.0 and >10^6.0 TCID₅₀ per animal, respectively. The lower immunogenicity of virus delivered by the ocular route compared with the intranasal route can be correlated with a lowered capacity of ocularly administered virus to replicate in the murine respiratory tract.