The neuronal nuclei (NeuN) antigen is increasingly being used as a specific marker to identify neuronal cell loss under various pathological conditions. However, recent studies pointed out that a decrease in NeuN labeling could also be due to the reduction of protein expression level or loss of antigenicity and this was not necessarily related to neuronal cell disappearance. We also investigated the presence of damaged neurons, the loss of NeuN immunoreactivity and the level of NeuN protein in the brain hippocampus of mice subjected to soman poisoning (1.2 LD50 of soman). Damaged neurons were detected using hemalun-phloxin (H&P) and Fluoro-Jade B (FJB) staining on brain sections. NeuN immunohistochemistry was also performed on adjacent brain sections and NeuN protein level quantified by Western blot analysis. One and eight days after soman exposure, about 49% of hippocampal neurons were damaged, as assessed by H&P or FJB staining. NeuN immunohistochemistry indicated that all these damaged neurons were deprived of NeuN immunoreactivity. Using Western blot analysis, we proved that loss of NeuN immunoreactivity in degenerating neurons was due to reduced NeuN antigenicity rather than a fall in protein expression level. In this study, we discuss the potential use of NeuN immunohistochemistry as a good biomarker to predict delayed neuronal degeneration in the rodent hippocampus after various brain injuries.