Effects of intratracheally administered air pollutant chemicals on susceptibility of mice to infection by aerosolized bacteria were compared with effects of the same chemical previously studied by the inhalation route. Out of 22 compounds previously studied by inhalation, the same 3 (CdSO4, CuSO4, and ZnSO4) that caused significantly increased mortality because of infection after a 3-h exposure to 1 to 2 mg/m3 showed large (greater than or equal to 40% over sham injected) mortality increases when administered by intratracheal injection at lung concentrations calculated to be equivalent to those achieved during inhalation exposures (0.6 to 1.2 microgram). Dose-effect curves of these compounds were similar by the 2 routes. Five of the compounds caused a moderate (20 to 30%) enhancement in the mean excess mortality at concentrations of 2 to 3 microgram/animal. The remaining compounds, which showed no significant effect by inhalation, also showed no effect when given by intratracheal injection. Thus, similar effects on susceptibility to lung bacterial infection were seen whether chemicals were deposited in te lung by inhalation or by intratracheal injection.