Hormones and other polypeptides secreted by cells are frequently biosynthesized as larger precursors. The necessary enzymic processing of these occurs prior to the secretion of hormones but after the secretion of blood factors and zymogens. These conversion processes are of particular importance in regulatory mechanisms [l] such as blood clotting. At present, the mechanisms of conversion from precursors to hormones are poorly understood in all but a few instances. Furthermore, hormones themselves are sometimes found as several homologous species, each with some activity, but differing in peptide length [2,3] . A clearer understanding of the intracellular processing events may help to distinguish between primary hormonal products and artefacts produced by non-specific enzyme attack [4] . A general mechanism for the intracellular processing of propolypeptides has been proposed. In this a trypsin-like enzyme cuts precursors specifically at adjacent basic ammo acids. The resulting basic COOHterminal residues are then removed by a carboxypeptidase B-like enzyme [5,6] . An example is the release of the C-peptide from proinsulin by cleavage at the carboxyl ends of -Arg-Argand -Lys-Argsequences. Both trypsin and carboxypeptidase B-like activities have been observed during the conversion of proparathyrin [ 161. It is not known whether these enzymes are the same in all cells producing secretory material, or whether they are specific to particular polypeptides. For example albumin, glucagon and parathyrin all retain paired basic amino acid sequences, although