Testicular germ cell tumours (TGCTs) are the most frequent solid tumour in young men (19 to 39yo). They arise from fetal germ cells that fail to differentiate and transform into tumours after puberty. TGCTs are classified into either seminomas, selectively marked by SOX17 expression, or non-seminomas, most of which express SOX2 but lack SOX17. Seminomas can be reprogrammed to the more clinically challenging non-seminomas in vitro and in vivo. Understanding the regulation of SOX2 and SOX17 may provide insight into germ cell tumour fate and cancer prognosis. This study, using TCam-2 cells, the established cell line representing seminoma, addresses the potential for the combined impact of an environmental chemical, mono-2-ethylhexyl phthalate (MEHP) and inappropriately elevated exposure to growth factor activin A to influence TGCT aetiology. The reprogramming response of TCam-2 cells was interrogated by exposure to these factors. Cells were cultured with activin A (5 ng/mL), MEHP (100µM), both combined, MEHP plus the potent activin/TGFβ/NODAL signalling inhibitor (SB431542) or vehicle controls for 48 hours (n=3) in serum-reduced conditions (2.5% FBS). Transcripts were measured by qRT-PCR. MEHP, activin A, and their combination each significantly upregulated SOX2 (1.37-, 1.41-, 1.42-fold, respectively). However, SB alone and MEHP plus SB decreased SOX2 (0.61-, 0.58- fold, respectively), suggesting MEHP and activin A work through the same pathway. SOX17, was unchanged by MEHP, but was upregulated (1.4-fold) by activin A, different from the outcome with SOX2. ETV5, encoding a transcription factor essential for maintaining spermatogonia stem cell self-renewal, was decreased by MEHP (0.54-fold), activin A (0.30-fold), and their combination (0.22-fold), indicating an additive effect. Cell migration assays and a non-seminoma model, NT2/D1 cells, will also be used to examine activin A and MEHP’s effects on cancer progression. These outcomes demonstrate how concurrent environmental and intrinsic factor exposure may worsen TGCT outcomes for young men.