Intraperitoneal inoculation with Japanese encephalitis virus (JEV) induces the generation of T suppressor cells for delayed type hypersensitivity (Ts-DTH) in Swiss albino mice. The Ts-DTH are hydrocortisone resistant, partially sensitive to X-irradiation and the membrane phenotype of Ts cells is Ly I+. Ts-DTH suppression is mediated through the production of soluble suppressor factor (SF-DTH). SF-DTH is non-dialysable but passes through 450 nm filter and does not sediment after centrifugation at 100,000 g for 2 h; chromatography on Sephadex G-100 indicates an approximate molecular weight of 12,000.