20 ICR mice were inoculated with Clostridium piliformis via tail vein. Clinical signs of Tyzzer's disease in infected mice were not observed. Gr oss lesion of liver with varying severity in 10 mice treated with cortisone acet ate was observed 4 to 7 days post-infection. Typical Clostridium piliformis on s tamp preparation of postmortem liver were seen in a “pick-up-stick” arrangemen t within or beyond hepatocytes with Giemsa stain. Bacterial culture on artificia l media was negative. Meanwhile no gross lesions and Clostridium piliformis were identified both in 5 sham control mice and 10 no cortisone treatment mice, The above results suggest that inducing gross lesion of liver and Clostridium pilifo rmis propagation are associated with immunological state of experimental animals .