Histopathology of lungs exposed to MWCNT displays granulomatous and fibrotic tissue at 30 days post-exposure. Mice instilled with (A) 10% saline in surfactant vehicle control display normal lung morphology while mice instilled with (B) 4 mg/kg MWCNTs exhibit widely dispersed deposition of MWCNT aggregates within lung tissue. Masson's trichrome staining shows collagen rich granulomas and surrounding fibrotic tissue (blue) in lungs of mice exposed to (D) MWCNTs, but not (C) vehicle control. H&E staining demonstrates granulomatous peribronchioloar foci in lungs of mice exposed to MWCNT (F) but not vehicle (E). Agglomerates of MWCNT within granulomas are indicated by arrows. Images are representative of 4 mice per group with original magnifications of 25 × (A-B), 200 × (C-D) and 400 × (E-F).