Recently, placental alkaline phosphatase (PLAP) has been suggested as a tumour marker in patients with seminomas (S), since elevated serum levels of PLAP were found with high frequency in these patients. The present immunoperoxidase study of 33 testicular germ cell tumours was undertaken to localize PLAP in the various types of these tumours as well as in the carcinoma-in-situ (CIS) pattern. Eighteen out of 19 (95%) S were PLAP positive compared to nine out of 14 (64%) non-seminomas (NS). In the NS the positive staining reaction was localized to tumour components of embryonal carcinoma (EC) in six cases, of choriocarcinoma (CC) in one and of S in two, while components of yolk sac tumour and teratoma were PLAP negative. The number of positively stained cells in S was much higher than in EC. The staining reaction was pronounced in the syncytiotrophoblast of CC and in some syncytiotrophoblast-like cells present in S. The staining reaction product was mainly confined to the cell membrane in the positive tumour types. In 20 out of 24 cases with CIS various numbers of CIS cells were PLAP positive, while PLAP was not found in normal germinal epithelium. Sixty three per cent of S patients had serum values above 1.0 micrograms/l, while such values occurred in 21% of NS patients. The tissue staining pattern for PLAP was found to correspond to the preoperative serum value. On the basis of these findings it is concluded that PLAP may be a useful marker in patients with S. Serum levels of PLAP may be used diagnostically in patients with testicular tumours and for monitoring therapy and detection of recurrences in patients with S. For optimal utility of this marker, determinations of serum profiles of PLAP are recommended. Finally, demonstration of PLAP in CIS indicates a functional relationship between CIS and S supporting the hypothesis that CIS is the precursor state of these tumours.