Castanospermine (CS) is an alkaloid isolated from the nut of the Australian chestnut tree, Castanospermum australae. It has been shown to have potent anti-inflammatory effects, as evidenced by its ability to inhibit the clinical manifestations of passively induced autoimmune encephalomyelitis and passive adjuvant arthritis in rats and the efferent phase of contact hypersensitivity in mice. The purpose of this study was to determine if CS has immunosuppressive, as well as anti-inflammatory, properties. Contact hypersensitivity in mice to picryl chloride was chosen as the in vitro model of cell mediated immune reactivity. Mice were sensitised with picryl chloride and treated with CS, at doses of 150 and 300 mg/kg/day, twice daily for seven days, beginning at the time of sensitisation. Passive transfer of spleen cells from treated animals transferred significantly less contact hypersensitivity (P<0.05) to naive mice than did cells from control mice. This suggests that CS inhibited the generation of picryl chloride reactive effector cells. This inhibition was not due to a depletion of the T cell phenotype responsible for contact hypersensitivity, CD4+, as FACS analysis showed no alteration in CD4+/CD8+ ratio in CS treated mice. In vitro studies, using antigen-specific cell lines, showed that CS inhibits antigen specific T-cell proliferation in a dose dependent fashion. Studies on the kinetics of this inhibition revealed that CS inhibits an early step, before 24 hours of culture have elapsed, in the T-cell proliferative response. Experiments were designed to examine if this early event was antigen processing by the accessory cells in the culture, or an early event in T-cell replication itself. CS not only failed to inhibit the antigen processing step, but when processing was carried out in the presence of CS the subsequent T-cell proliferation was enhanced. The results also indicated that CS, when added with preprocessed antigen, was inhibiting T-cell proliferation in a dose dependent manner. Subsequent studies examining the role of several key molecules in T-cell proliferation showed that CS did not effect the expression of the receptors for IL-2 or transferrin, nor did it alter the expression of the adhesion molecules LFA-1 or ICAM-1. The precise molecular mechanism by which CS inhibits contact hypersensitivity in vivo and T-cell proliferation in vitro still remains to be determined.