Interleukin-2 (IL-2) has been used extensively in cellular immunotherapy trials as a systemic activator of the immune system as well as an ex vivo stimulant for lymphoid cell function. Despite the measurement of several in vitro and in vivo immunologic parameters related to cellular immunotherapy, determinants of successful cellular immunotherapy remain unknown. To further delineate the consequences of exposing peripheral blood lymphocytes to high concentrations of IL-2, we assessed the supernatants of IL-2-activated peripheral blood lymphocytes for production of tumour necrosis factor (TNF) and interferon-gamma (IFN-gamma). Exposure of normal monocyte-depleted peripheral blood mononuclear cells (PBMC) to IL-2 caused a dose-dependent increase in secretion of TNF and IFN-gamma which increased linearly after 48 h in culture. Analysis of positively selected, highly purified PBMC subpopulations exposed to IL-2 revealed that TNF-alpha and TNF-beta were produced by both CD3+ and CD16+ subpopulations but not by CD22+ cells. These studies were extended to supernatants obtained from PBMC cultures used in the adoptive cellular immunotherapy of patients with advanced cancer. Patients treated with lymphokine-activated killer (LAK) cell immunotherapy were classified as responders (N = 14) or non-responders (N = 17) to therapy. We found no significant difference in the production of TNF between responders and nonresponders (22 +/- 9 U ml-1 vs. 20 +/- 6 U ml-1), P > 0.05. However, LAK cell supernatants harvested from non-responders contained a significantly higher level of IFN-gamma (232 +/- 94 U ml-1) compared with responders (42 +/- 14), P < 0.05. Furthermore, the linear association between IFN-gamma and TNF-alpha production was different between these two response groups (rs = -0.19 for non-responders and rs = 0.48 for responders). These results suggest that secondary cytokine production by adoptively transferred lymphocytesmay play an important role in the host response to cellular immunotherapy.