351 SPF OF1 non-immunized mice of both sexes were infected by aerosolized Pasteurella multocida. Female mice were significantly less susceptible to infection than were males of the same age (which were heavier). Males and females of equal bodyweight, but different age, showed no significant difference in susceptibility. The difference at equal ages can be explained by the lower respiratory flow of females. The consequences of these results for experimental studies on atmospheric pollutants are discussed.