Human testicular germ cell tumours of adolescents and adults (TGCTs), the seminomatous and non-seminomatous germ cell tumours, show morphological and biological similarities to normal embryonic development, presumably determined by their supposed cell of origin, the primordial germ cell/gonocyte. Based on this knowledge, OCT3/4, also known as POU5F1, was recently defined as a diagnostic marker for these tumour types. In the adult testis, positive immunohistochemistry for OCT3/4 is an absolute indicator for the presence of the TGCT precursor carcinoma in situ/intratubular germ cell neoplasia undifferentiated (CIS/ITGCNU), seminoma, and/or embryonal carcinoma. Several studies have confirmed this observation, using the same polyclonal antibody. The present study demonstrates the usefulness of OCT3/4 immunohistochemistry in a diagnostic setting of a consecutively collected series of more than 200 testicular tumours and over 80 testicular biopsies. Moreover, it is shown that a monoclonal antibody directed against OCT3/4 is as informative as the polyclonal antibody, both in immunohistochemistry and in western blot analysis. The antibodies are robust and applicable with different methods of pretreatment and storage of tissue. This allows routine application of this diagnostic marker.