It is well established that the induction of humoral or cellular response is influenced by the development of distinct subsets of CD4+ T cells.1 The Th1 cell subset produces predominately IL-2, GM-CSF, INF-γ, and TNF-â (type 1 cytokines) and is involved in delayed-type hypersensitivity reactions, whereas the Th2 cell subset secretes IL-4, IL-5, IL-6, IL-10, and IL-13 (type 2 cytokines), which are important factors for B cell growth and differentiation to Ig secretion. The imbalance of cytokine production by CD4+ T cells leads to a wide variety of immunological disorders, i.e. allergy, progressive lymphoproliferation, and severe immunodeficiency. Skin and lung biopsies from allergic patients indicate that the pivotal cells in the allergic site are the Th2 cells.2 Treatments effectively suppressing the function or the differentiation of these allergen-specific Th2 cells will most likely provide efficient ways to intervene in Ig-mediated allergic diseases. In the course of screening for chemical immunomodulators that inhibit the type 2 cytokine production in Th2 cells, we found cytoxazone (1) containing a 2-oxazolidinone ring, which is rare in microbial metabolites, as a novel cytokine modulator produced by Streptomyces sp.3 Cytoxazone (1) shows a cytokine-modulating activity by inhibiting the signaling pathway of Th2 cells, but not Th1 cells. We report herein mainly the structure elucidation of 1 based on NMR, CD spectra, and X-ray crystallographic experiments.