Increased collagen content in lung tissue of MWCNT exposed C57BL/6 mice. Collagen content was determined by lung tissue harvested from mice 30 days post-exposure to vehicle control (10% surfactant in saline) or MWCNTs (1, 2, or 4 mg/kg). Mice exposed to the high dose MWCNT (4 mg/kg) displayed significant increase in collagen levels compared to naïve mice. All values are expressed as mean ± SEM (n = 6-11). *p < 0.05 compared to naïve mice and + p < 0.05 compared between two groups. No significant differences were found between naïve and vehicle treated groups. In addition, differences between vehicle and treatment groups were not significant.