Oxidative stress or the production of reactive oxygen species (ROS) has been implicated as an important factor in the development of bleomycin (BLM)-induced pulmonary toxicity; however, the mechanism behind the toxicity remains to be elucidated. The present study aimed to investigate the key factor involved in BLM‑induced toxicity. The study was conducted in human pulmonary fibroblast (HPF) cells and in a mouse model. The ROS level, cell death assay, protein and gene expression levels of Kruppel‑like factor 9 (Klf9) and other associated factors were assessed. A dose‑dependent increase in ROS, lipid peroxidation, cell death, and protein and mRNA expression levels of NF‑E2‑related transcription factor 2 (Nrf2) and Klf9 were observed in BLM‑treated cells. However, the expression levels of the other antioxidant proteins assessed, including catalase, super oxide dismutase, glutathione reductase and thioredoxin reductase 2, were decreased. The expression levels of Nrf2 were decreased in cells treated with a higher concentration (>200 µM) of BLM. These results suggested that in response to increased intracellular levels of ROS, above a critical threshold, Nrf2 stimulates the expression of Klf9, resulting in a further increase in Klf9‑mediated ROS production and subsequent cell death. Furthermore, the data suggested that Klf9 may be considered as an adjunctive therapeutic target for BLM-induced pulmonary toxicity.