Immunocytochemistry of secretory peptides involves the localization of hormonal, neuronal, and paracrine messenger molecules at the cellular level. Immunological studies of these peptides have revealed them to be heterogenous at both the cellular, subcellular, ontogenetic, phylogenetic, and molecular level. It is probable that this heterogeneity reflects a diversity in function and that the same peptide backbone may be utilized for different tasks by different cells. One tentative way in which this could be brought about is by selective proteolytic, processing mechanisms. Although preliminary evidence indicates that such site-dependent processing occurs, much work remains to be done on the accurate localization of peptide fragments by specific, precise, sensitive, and selective tools. Recent progress in antibody production and evaluation as well as in immunocytochemical methodology has put such tools at our disposal. The present review discusses the use of these tools with particular emphasis on the interpretation of results, and their correlation to chemical and physiological information.