The mRNA expression of Th1 and Th2 cytokines was compared in freshly isolated CD3+ tumor-infiltrating lymphocytes (CD3+ TIL) and in autologous CD3+ peripheral blood lymphocytes (CD3+ PBL) obtained simultaneously from 20 patients with renal cell carcinomas (RCC). In addition cytokine expression was compared in CD4+ TIL and CD8+ TIL from another group of 20 patients with RCC. TIL were isolated from mechanically disaggregated tumor material and PBL from peripheral blood by gradient centrifugation and subsequent selection with anti-CD3, anti-CD4 or anti-CD8 magnetic beads. In these pure lymphocyte preparations the constitutive expression of interleukin-1 (IL-1), IL-2, IL-10, interferon gamma (IFNgamma), and tumor necrosis factor alpha (TNFalpha) was determined by using a polymerase-chain-reaction-assisted mRNA amplification assay. In the CD3+ TIL, levels of mRNA for IFNgamma, IL-10, IL-1 and TNFalpha were significantly higher than in the autologous CD3+ PBL whereas IL-2 expression was rather low and did not differ in the two populations. Comparison of cytokine mRNA expression in CD4+ TIL and simultaneously obtained CD8+ TIL revealed a significantly higher expression of IFNgamma in the CD8+ cells. These data reflect an in vivo activation of RCC-infiltrating lymphocytes at the mRNA level with respect to the Th1 as well as the Th2 immune response. Th1 activation seems to be most evident in the CD8+ TIL.