The intraperitoneal injection of Salmonella typhi into mice produced typhoid infection involving all the vital organs. Infection of liver was more persistent and progressive than in other organs. During the course of experimental infection, no humoral immune response was detected against phospholipase-A fractions upto 3 weeks after challenge, but significant cell mediated immunity (CMI) was found. Increased CMI response against protein antigens correlated well with the decreasing bacterial load, what suggested that CMI against proteins was important in pathogenesis of this disease.