Updated project metadata. Type II germ cell tumors (GCTs) are the most common neoplasia in young men of age 14-45 years. It is generally accepted that GCTs arise from a common precursor lesion, called germ cell neoplasia in situ (GCNIS), eventually developing into seminomas or non-seminomas, such as (embryonal carcinomas, ECs). The latter stem-cell like EC can further differentiate into teratomas (TE), yolk-sac tumors (YST), or choriocarcinomas (CC). Even though cisplatin-based chemotherapy is an efficacious standard of care during the management of GCT patients, the development of cisplatin resistance remains a major obstacle. As such, the surrounding tumor microenvironment and its secreted factors could endorse the development of drug resistance. These stromal cells, including immune cells (e.g. macrophages, T-lymphocytes), endothelial cells, and fibroblasts, can influence tumor cells by promoting proliferative and anti-apoptotic signaling pathways. Vice versa, microenvironmental cells can be influenced by tumor cells as well. For the identification of secreted proteins, cell lysates and supernatants from seven human germ cell tumor cell lines (seminoma: TCam-2; embryonal carcinoma (EC): 2102EP, NCCIT; choriocarcinoma (CC): JAR, JEG-3), yolk-sac tumor (YST): GCT72; 1411H) and five human cell types from the microenvironment (fibroblasts: MPAF, HVHF2; M2 macrophages: THP-1-M2; T-lymphocytes: JURKAT; endothelial cells: HUVEC) have been evaluated using liquid chromatography coupled with mass spectrometry (LC-MS).