Aims: The aim of this study was to demonstrate a prototype tool for measuring infectivity of an aerosolized human pathogen - influenza A/PR/8/34 (H1N1) virus - using a small-animal model in the Controlled Aerosol Test System (CATS). Methods and Results: Intranasal inoculation of nonadapted H1N1 virus into C57BL, BALB/c and CD-1 mice caused infection in all three species. Respiratory exposure of CD-1 mice to the aerosolized virus at graduated doses was accomplished in a modified rodent exposure apparatus. Weight change was recorded for 7 days postexposure, and viral populations in lung tissue homogenates were measured post mortem by DNA amplification (qRT-PCR), direct fluorescence and microscopic evaluation of cytopathic effect. Plots of weight change and of PCR cycle threshold vs delivered dose were linear to threshold doses of ~40 TCID50 and ~12 TCID50, respectively. Conclusions: MID50 for inspired H1N1 aerosols in CD-1 mice is between 12 and 40 TCID50; proportionality to dose of weight loss and viral populations makes the CD-1 mouse a useful model for measuring infectivity by inhalation. Significance and Impact of the Study: In the CATS, this mouse-virus model provides the first quantitative method to evaluate the ability of respiratory protective technologies to attenuate the infectivity of an inspired pathogenic aerosol.