Single wall nanoparticles (CN) are considered toxic with pulmonary effects that have been compared to asbestos. Our previous studies suggested only acute pulmonary toxicity by CN, but to evaluate synergistic effects this study compared acute (3 hr) intratracheal CN effects on rat lungs in animals with or without an ongoing “second hit” of an intraabdominal Staph. Aureus infection (1 week). Comparison groups: (1) CN only, (2) CN + infection, (3) infection only. Histopathology on 8 features of pulmonary tissue was scored. Lumen patency of pulmonary vessels was averaged. Macrophage (mφ) counts on CD68 stained sections and PMN counts on the three groups were made.Histopathology showed significant acute toxicity in lung septa (p=0.04), small bronchi (p=0.045) and pulmonary vessels (0.001). Lumen of lung vessels showed greater patency with nanoparticles (3 hr) than either group with infection, although the values had a p >0.05.Both macrophages (Mφ) and neutrophils (PMN) were elevated in lungs. The infection only group was highest, nanoparticles alone second highest, and nanoparticles plus infection had the lowest total cell counts, but group comparisons were below significance. We conclude that the initial steps to pulmonary toxicity by nanoparticle exposure are not exacerbated by the “second hit” of an ongoing distant infection in this model although acute toxicity was found histologically.Grant Funding Source: St Luke's Hospital foundation grant