Glucosamine (GlcN) and N-acetyl-d-glucosamine (GlcNAc) were assayed in vitro for their effects on proliferation, cytotoxicity and cytokine secretion in primary and secondary mixed lymphocyte cultures (MLCs). In addition, we studied the effect of GlcN and GlcNAc on the proliferation of purified CD4+ T cells exposed to immobilized anti-CD3 antibody. The present data show that GlcN, but not GlcNAc, inhibits CD4+ T-cell proliferation, the generation of alloreactive cytotoxic T lymphocytes (CTLs) and the secretion of interferon-gamma (IFN-gamma) and interleukin-5 (IL-5) in primary MLC. In secondary T helper-2 (Th2)-polarized MLC, GlcN, but not GlcNAc, inhibits IL-4 and IL-5 secretion, whereas no effect was found on IFN-gamma secretion in Th1-polarized MLC. Dendritic cells treated with GlcN showed a 75-80% decreased capacity for antigen cross-presentation and allostimulation. In cellular bioassays, GlcN was shown to inhibit the stimulatory activity of IL-4 and IL-2, as well as the cytotoxic activity of tumour necrosis factor-alpha (TNF-alpha). In conclusion, GlcN suppresses unprimed T-cell responses by interfering with antigen-presenting cell functions and by a direct inhibitory effect on T-cell proliferation. In addition, GlcN inhibits the secretion of cytokines in antigen-stimulated unprimed T cells and primed Th2-polarized cells.