Testicular carcinoma in situ (CIS) is the precursor of germ cell tumours in adults, except for spermatocytic seminoma. The mechanism of the progression from premalignant CIS to invasive and overt tumours is largely unknown. There are currently two main hypotheses: one is that CIS can progress directly to either seminoma or nonseminoma; according to the other, seminoma is the intermediate stage between CIS and nonseminoma. CIS cells express several tumour antigens, such as placental-like alkaline phosphatase (PLAP), TRA-1-60, or the c-kit proto-oncogene protein product (Kit), which are present to varying degrees in the invasive germ cell tumours. In this study, CIS cells adjacent to either pure or combined tumours were examined by double immunohistochemical staining for simultaneous expression of TRA-1-60 (typical for embryonal carcinoma) and either Kit (expressed by seminomas) or PLAP (found mainly in seminomas, but also in some cases of nonseminoma). Marked differences in the expression of these antigens were found among CIS cells, especially those adjacent to mixed tumours. We conclude that CIS is a phenotypically heterogeneous lesion, and that the CIS cells, despite identical morphology and close spatial localization, may be in different stages of progression. The results lend support to the hypothesis that CIS can progress directly to both seminomatous and nonseminomatous tumours.