Abstract We developed a working hypothesis and relevant models based on it, which can be applied to investigations of immunoregulatory processes. Oligopeptides effecting signalling functions (activators or supressors of lymphocytes, macrophages, and other immune system cells) are postulated to emerge from protein precursors by limited proteolytic cleavage close to the appropriate cell receptors (the distance between the sites of formation and action of peptides is comparable with the cell dimensions). The limited proteolysis reactions presumably occur during cooperation of immunocompetent cells involving a mutual (tete-a-tete) exchange of information between lymphocytes or lymphocytes and macrophages; the resulting oligopeptides (called tetines or cellular hormones) and/or their precursors are transferred by direct contact via immunosynapsis from cell to cell. We used the signature and equivocation principles of information theory, as well as data on the quasicyclic structures of active sites of peptides and proteins, to further our knowledge of consecutive limited proteolysis and quasicyclization reactions in conjunction with the processes of tetine formation. New groups of immunologically active peptides identified on the basis of the theoretical concepts and models outlined above include rigin and escarines (macrophage regulators), immunopoietins (T-cell regulators), polarins (a tetine subgroup composed of polar amino acid residues eliciting a wide spectrum of biological responses), and IgG and IgE C-terminal peptides (putative cytophilic active sites responsible for degranulation of mast cells and basophils with subsequent release of inflammation mediators). All of these peptides comprise, on the average, three to nine amino acid residues, with their biological effects being manifest in concentrations characteristic of peptide hormones and showing dependence on chemical structure. Available experimental evidence leads us to suggest that immunoglobulins, being polyfunctional proteins, may also perform the functions of prohormones (protetines), which are primarily ascribed to the C-terminal domains of the molecules. Since structurally and functionally similar peptide fragments occur, apart from immunoglobulin molecules, in interferons, thymic hormones, growth factors, histones, neuropeptides, and other natural bioregulators, it can be postulated that tetines constitute a universal bioregulatory system. It is also believed that many natural polypeptides and proteins act by releasing oligopeptide fragments of the tetine type in direct proximity to cell receptors.