Human allergen-specific Th2 cells producing cytokines like IL-4, IL-10 are associated with atopic disorders, so inhibitors of Th2-dependent cytokine production are potential immunotherapeutic agents. In screening microbial metabolites, we isolated a novel cytokine modulator, cytoxazone, from Streptomyces sp. strain RK95-31 (isolated from Japanese soil, deposited as FERM P-16171). The strain was cultured, and cytoxazone was extracted from fermentation broth with EtOAc, purified via silica gel chromatography, HPLC, and recrystallization. Cytoxazone is a colorless crystal (m.p. 118-121℃, [α]23D-71°) with molecular formula C11H13NO4, structure identified as (4R,5R)-5-hydroxymethyl-4-(4-methoxyphenyl)-2-oxazolidinone. Biological assays showed cytoxazone (6.25-25μg/ml) significantly inhibited PWM-induced IL-4 and IL-10 production in BALB/c mouse spleen cells without affecting GM-CSF. It inhibited IL-4/IL-10 production from Th2 clone S11-7 but not GM-CSF from Th1 clone S38-9, with no growth inhibition at 100μg/ml. These results suggest cytoxazone inhibits cytokine production via Th2 cell signaling pathways, not Th1, and further mechanism studies are ongoing.