Recent years have seen enormous advances in the field of protein and peptide engineering and a greater understanding in the way in which biological response modifiers function in the body. It is now possible through the use of recombinant DNA techniques, or by solid phase protein synthesis, to produce significant quantities of a wide variety of regulatory agents that are therapeutically applicable. The list of these response modifiers expands almost daily to include interferons, macrophage activation factors, neuropeptides and agents that may have potential in cardiovascular disease, inflammation, contraception etc. Prospects to use some of these materials in medicine have reached the stage where products have either been approved by regulatory authorities or are the subject of applications as investigatory drugs or as new therapeutic agents. In some uses the pertinent agent will be administered on an acute basis in the form of a simple injection, as, for example, the use of a tissue plasminogen activator for the treatment of coronary infarct. In other cases regulatory proteins and peptides are indicated for chronic therapy and here they will need to be administered by an appropriate delivery system. Unfortunately, the research on delivery systems for peptides and proteins has not kept pace with the rapid progress in biotechnology and, consequently, there are presently few systems that are entirely appropriate for the administration of macromolecular drugs according to complex dosage regimens, (eg intermittent and pulsed therapy). Furthermore essential pharmacokinetic and pharmacodynamic data may be missing.