Ninety-six Macaca mulatta were exposed to aerosol particles containing Pasteurella tularensis. Four different aerosols were employed that contained particle size distributions with median diameters of 2.1, 7.5, 12.5, or 24.0 mum. Size distributions were calculated only for those particles observed by phase microscope to contain organisms. Animals exposed to particles whose median diameters were either 2.1 or 7.5 mum were all infected and showed extensive infection of the lower respiratory tract, evidenced by large patches of consolidation with many necrotic foci on the surface. Death occurred in these animals 4 to 8 days after exposure. Monkeys exposed to 12.5- or 24.0-mum median diameter particles presented involvement of the cervical and mandibular lymph nodes, evidenced by swelling and abscess formation. Thirty-eight of the 45 animals in this group were infected. Those animals succumbing to the disease died from 8 to 21 days after exposure. The respiratory LD(50) values increased from 14 to 4,447 cells as the median diameter was raised from 2.1 to 24.0 mum.