The DNA-cleaving, antitumor antibiotic bleomycin (BLM) causes pulmonary fibrosis, but the essential early events initiating the fibrotic state have not been well characterized. Thus, we have directly examined BLM-mediated pulmonary cell injury by monitoring lactate dehydrogenase (LDH) release and nuclear poly(ADP-ribose) polymerase (PAP) activity, which is stimulated by DNA breakage, using lung slices isolated from BLM-sensitive (C57B1/6) and BLM-resistant (BALB/c) mice. Lung slices were incubated continuously with or without the PAP inhibitor, 3-aminobenzamide (3-AB), and exposed to BLM for 45 min. LDH release from C57B1/6 lung slices increased 2-fold by 8.5 h after treatment with BLM. In contrast, BLM failed to enhance cumulative LDH release by BALB/c mouse lung slices. Co-incubation of C57B1/6 lung slices with 3-AB prevented BLM-induced LDH release. Nuclear PAP was activated 3- to 4-fold 1.25 h after exposure of C57B1/6 lung slices to BLM but returned to control levels by 3.75 h. Nuclear PAP was only marginally affected at these times in BALB/c lung slices. Co-incubation of C57B1/6 slices with 3-AB prevented the early increases in PAP activity. These results demonstrate that murine strain sensitivity to acute cell injury and early PAP activation by BLM in lung slices parallels the in vivo sensitivity of lungs. In addition, 3-AB suppresses PAP activation and acute cell injury in lung slices. Differential activation of PAP appears to govern murine strain variation in response to BLM and is consistent with the hypothesis that activation of PAP participates in acute pneumocyte injury, initiating the process of BLM-induced fibrosis.