NMRI mice were immunized intramuscular, intranasal or by Aerosol, using the ethylethylenimine inactivated and polyethylenglycolconcentrated influenza virus strain A/PR/8/34 (HO/N1). Differences in the immune response resulted from all three routes. Intranasal and intramuscular vaccination were superior to aerosol application. A possible explanation for this could be the fact that relatively small amounts of the inhaled virus antigen developed antigenic activity on the mucous membrane. A single vaccination by the aerosol technique gave significant protection only, if the challenge virus was applied by the same procedure. However no protection was found after intranasal challenge. Intranasal challenge on the third day post vaccination revealed that intramuscular immunization had a significant better protective effect than intranasal immunization. However from the 5th to the 10th day post vaccination this effect reversed and intranasal vaccination became superior. This immunity persisted for the whole period of observation and it was accompained by a higher titer of local antibodies. Similar results were obtained in experiments with aerosol challenge. Here only the intranasal vaccinated mice were completely protected after the 10th day post vaccinationem while intramuscular vaccinated animals were less protected. Sera of intramuscular immunized mice revealed a higher content in antibodies of the Ig M type and less of the Ig G type compared to mice vaccinated by the intranasal route.