The ultrastructure and cytochemistry of the stem cell and amoebocytic blood cells of the sea squirt, Ciona intestinalis, are described. The stem cells are characterised by a high nuclear: cytoplasmic ratio and on undifferentiated cytoplasm. In thin sections, four types of amoebocytes can be demonstrated: non-vacuolar and vacuolar hyaline amoebocytes, granular amoebocytes, and refractile amoebocytes. The cytoplasm of non-vacuolar amoebocytes contains many vesicles, which although structurally similar to primary lysosomes are in most cases acid phosphatase-negative. The granular amoebocytes enclose several microtubule-containing granules and vacuoles with electron-dense vanadium deposits, while the refractive amoebocytes are characterised by their pleomorphic inclusions and spike-like pseudopodia. Both the granules and inclusions of these amoebocyte types are peroxidase- and acid phosphatase-negative indicating that they are probably not lysosomes. The possible interrelationships of the four amoebocyte types are discussed, and it is suggested that the vacuolar hyaline amoebocytes may differentiate into granular amoebocytes.