Neuroendocrine cells and other secretory cell types are able to store secretory proteins in a concentrated form for extended periods until the release of large quantities of protein is triggered. The proteins are stored in dense core secretory granules. The dense cores of these granules are made up of large, insoluble aggregates that form by self-association. These aggregates solubilise rapidly into monomeric proteins in their native conformations when released from the cells by exocytosis of secretory granules. Formation of aggregates is an early event in secretory granule formation in at least some cell types. The function of secretory granules containing protein aggregates varies, depending upon the contents. This may occur because recognition of an aspect, such as a surface motif, of the aggregate facilitates correct assembly of the membrane proteins necessary for transport and exocytosis of the granules.Understanding the principles necessary for aggregation of protein hormones may help in the formulation of proteins for clinical use. Formation of aggregates of human prolactin has been investigated both in cells and in solution. In cells, the aggregation of human prolactin requires a mildly acidic pH, and is slowed in the presence of a membrane-permeable chelator of zinc. In solution, the aggregation of human prolactin at mildly acidic pH and physiological concentrations of Zn2+resembles that which occurs in cells if the reaction is performed with macromolecular crowding, which will mimic the conditions in cells. The factors causing protein aggregation and the extent to which aggregation plays a role in secretory granule formation are likely to vary with the protein and cell type. Further understanding of the principles involved in forming these aggregates that readily disassociate may enhance the ability to formulate protein preparations. Knowledge of the exact residues involved in the protein : protein interfaces in the aggregates of secretory granule proteins may lead to the ability to use small molecules to interfere with self-association and to regulate the storage of secretory granule proteins.