Coelomocytes (blood cells) of the solitary ascidian Halocynthia roretzi were placed in culture to which sheep red blood cells (SRBCs) were introduced. Their reactions against SRBCs were observed by phase-contrast, time-lapse video and scanning electron microscopy. Three cell types, the phago-amoebocyte, the vacuolated cell and the fusogenic phagocyte, reacted to the SRBCs by phagocytosis. The vacuolated cell occasionally discharged the material of their vacuoles at the time of or shortly after ingesting the SRBC. The lymphoid cell captured SRBCs, but did not ingest them. Three other cell types, the fibroblastic cell, the giant cell and the multipolar cell, showed no reaction to the SRBC. The SRBCs captured by the lymphoid cells were transferred either to phago-amoebocytes or fusogenic phagocytes, probably to be ingested. Such collaboration between multifunctional cell is considered to characterize the cellular defense mechanism of H. roretzi.