Heparin inhibits the antiparasitic and immune modulatory effects of human recombinant interferon-y * Interferon-y (IFN-y) is a potent immune regulatory cytokine and is, in addition, involved in the induction of antiparasitic effector mechanisms in different cell types. The first step of IFNy action is its binding to a specific receptor. Furthermore, it has been shown that IFN-y binds with a great affinity to the heparin-like structure of heparan sulfate, which is localized in basement membranes and on cell surfaces. In this study, we analyze the effect of heparin and heparan sulfate on three different IFN-y-mediated activities inducible in human glioblastoma cells (87HG31 and 86HG39). We find firstly that heparin is able to inhibit IFN-y-mediated induction of major histocompatibility complex (MHC) class I1 antigen expression on 87HG31 cells, an effect which can be abrogated by protamine. Secondly, we show that heparin inhibits the IFN-y-induced toxoplasmostasis within 86HG39 cells in a dosedependent fashion, and thirdly that heparin inhibits the IFN-y-mediated induction of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase. In contrast to IFN-y-induced effects, the activity of other cytokines, such as interleukin (1L)-1, I L 2 and IL-6, is not influenced by heparin. The possible mechanism of heparin-induced inhibition of IFN-y is discussed.