Most proteins undergo posttranslational modifications that govern the function of the protein. In synchrony, correspondingly unmodified proteins that are functionally silent or act differently may also be synthesized. The gut hormone precursor, procholecystokinin (proCCK) is an example of a protein that is heavily modified. An essential modification is O-sulfation of Y₇₇, which is necessary for the gallbladder emptying effect of CCK peptides via the CCKA-receptor. In order to examine possible in vivo synthesis also of nonsulfated CCK, we have established a two-step analysis that requires tryptic cleavage at a defined processing site in proCCK (R₇₅-D₇₆) followed by monospecific RIA-measurement of the then exposed nonsulfated N-terminal sequence of CCK-8 (DYMGW…). The analysis shows that endocrine cells in the gut synthesize nonsulfated CCK peptides (-58, -33, -22, and -8) in the order of 20-35% of the corresponding sulfated CCKs. Since nonsulfated CCK peptides are full agonists of the CCKB-receptor, the assay has revealed a hitherto unrecognized gut hormonal peptide system. The assay may prove useful in the diagnosis and control of diseases with hyperCCKemia. This includes CCK-producing neuroendocrine tumors such as the recently described CCKomas and medullary thyroid C-cell carcinomas.