Perforin is known as a pore-forming cytotoxic granule released from cytotoxic T cells. Previous experiments in vitro revealed the presence of precursor cells that are capable of producing perforin in the immune system cells. The present study was undertaken to examine whether perforin-positive cells could be induced in the digestive tract and to characterize their precursor cells. Expression of perforin-positive cells in the intestine of Balb/c mice induced by OK-432 was analyzed by immunohistochemical staining and RT-PCR. Oral treatment of Balb/c mice with OK-432 resulted in the occurrence of perforin-positive cells in the inferior segment of small intestine, the superior segment of large intestine, mesenteric lymph nodes and spleen. In the small intestine, perforin-positive cells were found in the lamina propria mucosa. The presence of perforin-positive cells was also noted following long-term OK-432 treatment. Similar results were obtained following treatment with biological response modifiers such as lipopolysaccharide. In mice with GVHD (graft-versus-host disease), the presence of perforin-positive cells was noted in the small intestine and spleen. When the serial sections of the small intestinal mucosa from OK-432-treated mice were immunostained with anti-perforin, anti-CD8 and anti-asialo-GM1 antibodies, the perforin-positive cells were found to be CD8-positive. The results suggest that CD8(+) cells in lamina propria mucosa play a significant role as effectors in the mucosal immune system which is activated by various stimuli.