In this study, we conducted a phosphoproteomic analysis of two cell lines with differing monoclonal antibody (mAb) production levels using label-free quantification mass spectrometry (LFQ-MS). The high-producing cell line, 353/11, was compared to the lower-producing 2G12 cell line. Phosphoproteomic samples were collected on day six of cell culture. To ensure synchronized sampling, all samples were harvested four hours after medium exchange. Additionally, 353/11 cells were separately cultivated and treated in parallel with 1 µg/ml tunicamycin (TM) for four hours to induce mild endoplasmic reticulum (ER) stress.