The c-kit proto-oncogene product and its ligand stem cell factor play an important role in haematopoiesis, spermatogenesis, and melanogenesis. Using an anti-c-kit antiserum raised against a synthetic peptide, we studied the immunohistochemical expression of the c-kit gene product in 60 germ cell tumours (GCTs) (53 testicular, 7 extragonadal), derived from primary GCTs in 45 cases and metastatic tumours in 15 cases. Twenty-eight out of 28 seminomas showed c-kit membranous staining in the majority of cells. A similar pattern of expression was seen in intratubular germ cell neoplasia. Nine out of 29 (32 per cent) non-seminomas displayed cytoplasmic, but not membranous, c-kit immunoreactivity in occasional cells. In three mixed GCTs, c-kit expression was limited to the seminoma component. In normal testis, c-kit expression was observed in some basal tubular cells, corresponding to undifferentiated spermatogonia. These results suggest a role for c-kit in the oncogenesis of GCT, where down-regulation of c-kit might be a critical step during progression from seminomas to non-seminomas. Immunohistochemical analysis of c-kit should be considered as a diagnostic aid for GCT and in particular may be helpful in the identification of certain extragonadal seminomas.