We investigated the prophylactic and therapeutic efficacy of an intravenous (IV) formulation of zanamivir in a macaque infection model for highly pathogenic influenza A (H5N1) virus. Antiviral efficacy was dose-dependent, with no reduction in viral load observed at 2 mg/kg, but a significant reduction observed at 10 mg/kg (p=0.039) and at 20 mg/kg in the combined prophylactic and therapeutic groups (p=0.049) with both prophylaxis (commencing 12 h before infection) and therapy (commencing 4 h after infection) showing similar reductions in viral load. Combined gross pathology and microscopic pneumonia scores in the treated animals relative to untreated controls were significantly reduced at 10 mg/kg (p=0.02) and at 20 mg/kg in the prophylaxis group (p=0.02), but were not significant in the treatment group (p=0.145). In this new animal model for evaluation of influenza antivirals, despite variability observed between individual animals, IV zanamivir showed evidence of efficacy against highly pathogenic H5N1 virus.