Although we know a great deal about vertebrate immune mechanisms, studies on the origins of such activity, which surely must reside among the invertebrates, are only in their infancy. We are proposing in depth studies on the immune capabilities of the sea star Dermasterias imbricata since: It demonstrates adaptive cell-mediated immunity; it represents an advanced invertebrate thought to be directly ancestral to the vertebrates; it is quite hardy in captivity and totally amenable to experimental procedures. We propose extensive grafting studies to determine if animals at this phylogenetic level can generate long-term immunological memory. Histological and ultrastructural studies of allograft tissues, in various stages of rejection, will be carried out in order to clearly define the population(s) of effector cells mediating the response. Antigenically stressed animals will be surveyed for the possible existence of organized lymphoid-like tissue. In vitro assays will be employed to determine if sea star hemocytes can demonstrate mitogenic and mixed lymphocyte reactivity characteristic of vertebrate lymphocytes. We will also search for the existence of humoral factors analogous to those prevalent in higher vertebrates, such as inducible antibody-like factors and possible complement-like factors. Preliminary attempts at defining the nature of cell surface receptors will be undertaken by using immunofluorescence techniques to search for the presence of beta2-microglobulin on sea star hemocytes. The data from these investigations should shed a great deal of light not only on the definitions of invertebrate immunity, but also on the evolutinary transition from invertebrate to vertebrate immune reactivity.