Direct diagnosis of swine influenza infection by an indirect immunofluorescence technique using anti‐nucleoproteine monoclonal antibody was compared with virus isolation. Five 8‐week‐old pigs were inoculated with 2 × 107 EID50of strain A H1N1Sww//4115/85. Clinical signs developed in only three pigs. Antigen was detected in nasal epithelial cells obtained from all animals the first day after inoculation; the antigen was detected in one pig 6 days after the infection. Fluorescence was present in the nucleus, nucleolus and cytoplasm of infected cells. The indirect immunofluorescence test was specific and as sensitive as virus isolation in embryonated eggs, allowing a rapid diagnosis that could be achieved within hours.