Sponge immunocyte identification is of interest to comparative immunologists since characterizing these cells will allow investigations into the mechanisms of non-self recognition in the oldest animal phylum. Here, we report that polyclonal antibodies raised against the core protein of a proteoglycan involved in cell adhesion in the marine sponge Microciona prolifera are specific markers for archaeocytes, the totipotent sponge cells. Archaeocytes are mobilized upon allogeneic contact and they accumulate in the contact zone. A second type of cell, the gray cells, are specifically recognized by monoclonal antibodies raised against CD44, a hyaluronan receptor. Gray cells do also accumulate in the contact area. Specific staining of a third sponge cell type, the rhabdiferous cells, shows that these do not accumulate upon allografting. These specific cell markers allow tracking of archaeocytes and gray cells, and show that they play an active role in sponge allogeneic reactions.