The teratocarcinoma-derived growth factor-1 (TDGF-1) gene codes for a 188-aminoacid glycoprotein that shares structural homology with the epidermal growth factor (EGF) family of growth factors. TDGF-1 is highly expressed in the undifferentiated embryonal carcinoma stem cell line NTERA2 clone D1 (NT2/D1) and its expression is downregulated in response to differentiating agents such as retinoic acid (RA) and hexamethylen-bisacetamide (HMBA). To assess the role of TDGF-1 in the onset and/or progression of human germ cell tumors, we analysed TDGF-1 expression by Northern blot and immunostaining in a panel of 59 human germ cell tumors of different histological origins. We show that TDGF-1 expression is markedly elevated in a subset of human testicular germ cell tumors as compared to normal testes. TDGF-1 overexpression occurs in about 100% of tumors with non-seminomatous phenotype, such as embryonal carcinomas and malignant undifferentiated teratocarcinomas. To address the questions of how TDGF-1 (previously called CRIPTO) may affect the growth and/or the differentiation of embryonal carcinoma cells, we have characterized the effects of exogenous recombinant TDGF-1 protein on the proliferation rate and differentiation potential of NT2/D1. Exogenous TDGF-1 protein stimulated DNA synthesis and cell proliferation in both undifferentiated and differentiated NT2/D1 cells. However, TDGF-1 protein treatment was unable to block differentiation induced by both RA and HMBA. These results suggest that TDGF-1 growth factor may represent an autocrine growth factor that may be involved in the process of development of testicular neoplasms.