Electron microscopic studies on the encapsulation induced by erythrocyte injection into the tunic of the ascidian Ciona intestinalis were carried out. The observations reported in the present paper complete the description previously given of capsule architecture and contribute to the characterization of the cells involved in the inflammatory reaction. The inflamed area is surrounded by an ample and peculiar "three-layered coat" respectively composed of flattened and packed extratunical hemocytes, the monolayered epithelium, and a layer of intratunical electron-dense particles. The latter are also clustered, variously arranged, and distributed in the tunic ground substance. The epithelial cells appear to be undergoing an active secretory phase; in several regions they also show discontinuities and organule and surface changes. The infiltrating hemocytes, mainly globular and unilocular granulocytes, appear frequently to be degranulating and in relation with electron-dense particles, net-like fibrous materials, and fine membranes. The involvement of morula-shaped granulocytes is discussed, as well as possible relationships with the melanization process, and finally analogies in the structural organization with the inflammatory reactions induced in other invertebrates. A schematic drawing, based on all available observations of the capsule architecture, is presented in order to reconstruct the entire inflamed area and illustrate the relative fine features.