We report that mast cells can bind and present IFN-γ in a functionally active form to macrophages. Flow-cytometric analysis revealed that biotinylated IFN-γ bound equally well to purified peritoneal mast cells from both IFN-γR knockout and wild-type mice, indicating a non-IFN-γR binding site. Purified peritoneal mast cells, loaded with IFN-γ for 30 min and washed, were able to induce NO synthesis by peritoneal macrophages. This response required cell contact and expression of IFN-γR on the responding macrophages, but not the mast cells. Human HMC-1 mast cells were also able to present IFN-γ to mouse macrophages. Enzyme treatment of mouse mast cells revealed that binding of IFN-γ was predominantly to chondroitin sulfate B (dermatan sulfate). Binding of IFN-γ to dermatan sulfate was confirmed by inhibition ELISA. This study demonstrates for the first time that mast cells can present IFN-γ to other cells via glycosaminoglycans. Mast cells may act as a reservoir of surface-stored functionally active cytokines.