Background/Aim: Caveolin, which is a major constructive component of the caveolar membranes, plays a key role in transcytosis of molecules into cells and regulation of several signal transductions. Caveolin has three isoforms, and recent studies suggest that in some malignant tumors an alteration of caveolin-1 expression correlates with oncogenetic changes. Caveolins have been reported to be negative regulators of inducible nitric oxide synthase (iNOS) which can provoke an antitumor response via infiltrating immune cells. The aim of this study was to examine the expressions of caveolin-1 and caveolin-3 (which is a caveolin-1 homologue localized predominantly in muscle tissue) in testicular cancer. Methods: We evaluated the expressions of caveolin-1, caveolin-3, and iNOS in 16 seminoma and 10 non-seminoma testicular cancer specimens as well as normal testicular tissue, using a streptavidin-biotin bridge technique on cryostat sections. The expression of caveolin-3 was confirmed by slot-blot analysis. Tumor-infiltrating immune cells were also studied immunohistochemically, and the correlation between the number of immune cells and caveolin-3 concentrations was calculated. Results: Immunohistochemistry revealed that caveolin-3, but not caveolin-1, was frequently expressed in seminomas (12 positive out of 16 specimens) without positive staining in their normal counterparts or in nonseminomatous germ cell tumors, except for muscle components in the teratoma. iNOS was not expressed in any tissues examined. Samples with high levels of caveolin-3 tended to have higher degrees of tumor-infiltrating immune cells, although this finding was not statistically significant. Conclusion: This is the first report demonstrating the involvement of caveolin-3 in germ cell tumors. Copyright (C) 2002 S. Karger AG, Basel.