The present review considers the structure, site of formation and functions of protochordate blood cells/coelomocytes. Details of the circulatory and coelomic system are also given where appropriate. In the hemichordates, blood cells are absent but a single type of amoebocyte is present in the coelomic cavity. A similar situation exists in cephalochordates and these cells appear to play an important role in maintaining the animal free from infection. In the urochordates, several morphologically distinct types of blood cells (haemocytes) exist, some of which store vanadium. At present little is known concerning the phylogeny and ontogeny of the different cell types in these groups.