Normal-density neutrophils (NDNs) isolated from patients with non–small cell lung cancer (NSCLC) inhibit T lymphocyte proliferation. To identify cancer-associated specificities of NDNs, we compared the proteome and phosphoproteome of NDNs from NSCLC patients with those from healthy donors. While the two populations were largely similar at the proteomic level, significant differences were observed at the phosphoproteomic level. Our results also indicate that NDNs are rapidly activated in vitro. Moreover, although coculture with NDNs induces only limited changes in the transcriptome of CD8 T lymphocytes, it is associated with a decrease in their proliferation. The second part of this work aims to elucidate the mechanisms by which neutrophils inhibit T lymphocyte proliferation.