Specific immunological recognition is not solely a vertebrate trait. Many invertebrates exhibit adaptive histocompatibility responses that bear considerable similarity to venebrate immune reactions. According to our postulate the adaptive immune systems of higher (vertebrate) animals evolved directly from more primitive systems of recognition that are still expressed among advanced invertebrates. If this hypothesis proves to be true, invertebrates may assume an important role as model systems for the analysis of fundamental immunological mechanisms that remain disguised by the complexity of vertebrate systems. Before such invertebrate models can be applied to the rationalization of complex problems, the evolutionary homology of vertebrate and invertebrate immunorecognition must be confirmed. Past studies have demonstrated substantial physiological similarities between invertebrate and vertebrate immunity. However, such functional similarities might also reflect the convergent evolution of otherwise unrelated systems. Convergent evolution would seem unquestionable if we consider that different phyla are included in «advanced invertebrates». Only detailed molecular comparisons will confirm true homology. Tunicates represent the closest extant ancestors of the vertebrates; they therefore have the greatest chance of yielding identifiable molecular homologies with vertebrate immune-related molecules. Currently we are analysing the immune system of tunicates with the view toward identifying such homologies. Our work does not assume that invertebrate recognition structures share similarity with their vertebrate equivalents. Instead, we plan to identify tunicate recognition structures in the context of their native system. This approach has a greater chance of success in fulfilling the goal of identifying invertebrate immunorecognition molecules.