The underlying cause for the observed poor antitumor activity of lymphocytes resident, in situ, in freshly resected human renal cell carcinoma (RCC) is unknown. To examine the basis for this observation we evaluated 217 T cell clones established from 20 consecutive patients with renal cell carcinoma. Of these, 75% were CD4+, 22% were CD8+, and 3% were NK+. Cytotoxic T cell function of CD4+ and CD8+ T cell clones was assessed by antibody-induced triggering of the TcR/CD3 complex. Nearly 93% of the CD4+ clones possessed poor cytolytic activity defined as <50% killing (E:T, 50:1). Alternatively, nearly 80% of CD8+ clones possessed strong cytolytic activity defined as ≥50% killing. In all T cell clones tested, MHC nonrestricted killing against Daudi or K562 was not observed. Overall, 77% of the T cell clones isolated from patients with RCC in this study were characterized by poor antitumor cytotoxic function; only 22% of the clones displayed strong cytolytic activity. Clones with strong cytotoxic function were tested for cytotoxic function against autologous tumor. However, all clones tested demonstrated little to no tumoricidal activity against autologous tumor. These results indicate that cytolytic T cells, while present in RCC, are not effectively activated by tumor to express CTL function. The immunobiology of CD4+ T cell clones was further evaluated. Thirty-two CD4+ clones from seven patients revealed distinct patterns of cytokine production following TcR/CD3 activation. CD4+ clones could be divided into IL-2 nonproducers and IL-2 producers. A total of 27/32 clones did not produce IL-2 following activation and IFN-γ production by IL-2 nonproducers was more than threefold less than that by IL-2 producers. However, no differences were found in the levels of IL-4, IL-6, GM-CSF, or TNF-α between the two groups. Antitumor cytotoxicity mediated by CD4+ clones did not correlate with cytokine production. These results demonstrate that among T cells resident in human RCC, the predominant type of lymphocyte population consists of noncytolytic helper CD4+ T cells capable of secreting IL-4, but importantly not IL-2, and low levels of IFN-γ following activation, thus resembling murine Th2 cells. Only a minor contribution by Th0-like cells was observed (i.e., CD4+ clones capable of secreting IL-2, IL-4, and IFN-γ following activation). These results suggest that the immunologic failure to contain RCC is related to the infiltration of T cell subsets, the majority of which resemble murine Th2, which cannot provide T cell help for the generation of CTL responses. In addition, although a small fraction of resident lymphocytes are CD8+ T cells that possess cytolytic function, antigen receptor triggering of this cell population by autologous tumor is not an effective stimulus for the induction of cytotoxic effector cell function.