Carcinoma-in-situ of the testis (CIS) is the precursor of invasive germ cell tumours. It is believed that CIS cells may originate from early fetal gonocytes. Recently, the proto-oncogene c-kit has been implicated as crucial for the development and migration of primordial germ cells. In this study, CIS and overtly invasive human male germ cell tumours were analysed immunohistochemically for expression of the c-kit proto-oncogene protein product. Testicular tissue samples from 36 patients with various types of testicular germ cell neoplasia and 19 control specimens were stained using an indirect immunoperoxidase method. High expression of c-kit was found in almost all cases of CIS, both when the lesion was the only pathology, and when CIS was adjacent to invasive tumours. The Kit staining was retained in seminomas with variable intensity; the majority of cells in tumour mass exhibited c-kit expression in 61% of the samples while focal expression was observed in 39% of the samples studied. No expression of c-kit was detected in non-seminomas or in normal testicular germ cells. High expression of the proto-oncogene in CIS cells supports the hypothesis of their origin from primordial germ cells. In addition, we propose that the c-kit protein product is a new marker for carcinoma-in-situ of the testis.