Human peripheral blood mononuclear cells (PBMCs) were cultured in the presence of different glutamine concentrations (0, 0.1, 0.4, 0.6, 2 mM) and stimulated with concanavalin A (Con A) or bacterial lipopolysaccharide (LPS). The concentrations of T lymphocyte- and monocyte-derived cytokines were measured in the culture medium 24 h later. The availability of glutamine significantly increased the production of interleukin (IL)-2 (2-fold), IL-10 (4-fold) and interferon (IFN)-gamma (4.5-fold) by Con A-stimulated PBMCs. Maximal production of these cytokines occurred at 0.1 mM glutamine and increasing the concentration of glutamine above that did not lead to a further increase in cytokine production. Glutamine availability resulted in small increases (24 to 35%) in the production of IL-1alpha, IL-1beta, IL-6 and tumour necrosis factor (TNF)-alpha by Con A-stimulated PBMCs; again maximal production occurred at a glutamine concentration of 0.1 mM. Glutamine availability did not influence the production of IL-1beta or TNF-alpha by LPS-stimulated PBMCs, while there was a small increase (17 to 32%) in the production of IL-1alpha, IL-6 and IL-10 by these cells at a glutamine concentration of 0.1 mM. It is concluded that glutamine enhances the production of T lymphocyte-derived cytokines with only minimal effects on production of cytokines by monocytes.