We previously showed that lectin strongly agglutinating human type A erythrocytes were present in the body surface mucus, playing an important role in host defense. However, it is unknown whether cellular and humoral defense factors are present in the protocoel of planaria. In this study, the presence of blood cells was first investigated by observing cells which flowed out of a cut surface of planaria. Results showed that there were several kinds of hemocytes; granulocytes, amoebocytes, hyaline cells, spherulocytes, red granule-containing cells, giant cells and eye-like cells. Granulocytes showed the most rapid cytolysis within one minute after bleeding, although other hemocyte types were relatively stable for over 10 min. An injection experiment also revealed that granulocytes phagocytosed foreign materials such as latex particles or human erythrocytes. Even stronger granulocyte phagocytosis was caused by treating erythrocytes with body surface lectin. Examination of the protocoel showed that it contained a cytotoxic humoral element of hemolysin or bacteriolysin. This factor was heat-labile and trypsin-resistant, and required Ca 2 + to elicit cytotoxic activity. SDS-PAGE analysis of the cytotoxic factor adsorbed onto a ghost revealed that 125kd protein adhered to the erythrocyte membrane. These results suggested that the defense system in the protocoel of planaria was constituted of at least cellular phagocytosis and humoral cytotoxic factor.