It has been previously reported that both short- and long-lived reactive oxygen species (ROS) are present on the surface of freshly generated fumes. The objective of this study was to determine if freshly formed welding fume induces greater lung inflammation and injury in rats due to the presence of reactive oxygen species than aged welding fume. Fume was collected during gas metal arc welding using a stainless steel consumable electrode and found to be of respirable size with a mean diameter of 0.77 microm +/- 0.48. Male CD/VAF rats were dosed intratracheally with the welding fume 30 min (fresh) and 1 and 7 days (aged) after fume collection at a dose of 1.0 mg/100 g b wt. Bronchoalveolar lavage (BAL) was performed 24 h post-instillation. Lung injury and inflammation were assessed by measuring the concentration of neutrophils, albumin, lactate dehydrogenase (LDH), and glucosaminidase (GLU) in the recovered BAL fluid. More neutrophils and enhanced GLU activity were observed for the 'fresh' group as compared to both 'aged' groups (P < 0.05). Slight, but not significant, elevations were seen in albumin content and LDH activity for the 'fresh' group as compared to the 'aged' groups. No significant differences were observed for any of the parameters when fume aged for 1 and 7 days were compared. When the 'fresh' and 'aged' fumes (12.5, 25, and 50 microg/ml) were suspended in dichlorofluorescin (15 microM), a probe which becomes fluorescent when oxidized, the concentration-dependent increases in fluorescence were greater for the 'fresh' fume versus the 'aged' fumes. We have demonstrated that freshly generated stainless steel welding fume induces greater lung inflammation than 'aged' fume. This is likely due to a higher concentration of ROS on fresh fume surfaces.