The levels of active and latent transforming growth factor beta 1 (aTGF-beta 1, ITGF-beta 1, respectively) in plasma samples were measured by enzyme-linked immunoabsorbent assay (ELISA). Samples were collected from patients suffering from renal cell carcinoma (RCC) before they underwent tumour resection. In all cases tested, the levels of latent TGF-beta 1 were much higher (n = 23, 41.0 +/- 13.9, range 19.3-78.1 ng/ml) than in healthy controls (n = 21, 3.8 +/- 2.9, range 0.6-9.9, P < < 0.0001), while active TGF-beta 1 did not differ that impressively (n = 38, 1.2 +/- 1.3, range 0.0-4.5 for RCC, n = 21, 0.1 +/- 0.2, range 0.0-1.1 in controls, P < 0.001). As for TGF-beta 1 production by proximal tubulus cells has been shown, it was speculated that these high TGF-beta 1 levels might be due to secretion by the tumour, which usually originates from proximal tubuli. Indeed, production of TGF-beta 1 was found in culture supernatants, and it was possible to show TGF-beta 1 mRNA expression in tumour samples. This TGF-beta 1 was predominantly secreted in the latent form. This is in contrast to data from other authors, who found TGF-beta 1 to be secreted mainly in the active form, but need not mean that it is inactive locally as the low pH encountered within tumours is in the range necessary for its activation. It is concluded that elevated latent TGF-beta 1 is common in RCC, is at least partially produced by the tumour and might be a cause for local immunosuppressive effects within the tumour.