Interferon gamma (IFN-gamma) has been shown to inhibit many of the activities of IL-4, including the induction of IgE synthesis and the proliferation of T cell clones. Here we demonstrate that IFN-gamma is able to inhibit the expression of IL-4 receptors on peripheral blood lymphocytes from both normal healthy donors and from patients with chronic lymphocytic leukaemia. Inhibition was shown to be dose-dependent and did not affect the binding affinity of the receptor as shown by Scatchard analysis. IFN-gamma was unable to displace labelled IL-4 from its membrane receptor, which demonstrates that IFN-gamma and IL-4 do not compete for the same membrane binding protein. The ability of IFN-gamma to down-regulate IL-4 receptors may be important in controlling certain immune responses.