The small granular type cells of coelomocytes from earthwarm reveal a cytotoxic activity against human NK target cell, K562. This killing process requires cell to cell contact same as human NK cells. However, killing mechanisms of foreign tumor cell is not fully characterized. In this study, we have examined perforin gene transcript and its protein in the ceolomocytes. We employed immunohistochemistry and Western blot analysis using mouse perforin antibody. RT-PCR and Southern blot analysis were performed by perforin specific primer and probe synthesizing based on the human perforin cDNA sequence.Perforin antibody reacted with the cytoplasmic granules in only small cell type of coelomocytes, but not in large cell type. These coelomocytes also reacted with Thy-1 antibody. Molecular size of earthwarm perforin analized by Western blot showed the size around 70 kDa. The perforin mRNAs from coelomocytes, mouse spleen cell and human PBL were revealed the same size which identified by RT-PCR and Southern blot analysis. These data indicated that perforin identified in the coelomocytes of earthwarm plays an important role for NK cytotoxicity similar to human and mice. Thus, earthwarm coelomocytes is a useful model to examine cellular immune defense mechanisms such as NK cell activity.