Protein has been reported to be the most satiating of all macronutrients. Upon gastrointestinal digestion, peptides are generated that stimulate the release of satiety hormones such as cholecystokinin (CCK) from enteroendocrine cells. As such, bioactive peptides could be the target of Functional Food ingredients with satiating effects. We set up an in vitro assay system to investigate if different protein hydrolysates exhibit varying CCK-releasing properties. Soy, pea, potato, casein, and whey protein hydrolysates were incubated with the enteric endocrine cell line STC-1 that endogenously expresses and secretes CCK. Release of CCK was measured by ELISA. All hydrolysates induced CCK release at low concentrations (>0.1 mg.L -1)); however, no significant differences in CCK-releasing properties between the different protein hydrolysates were found, suggesting a generic, nonspecific peptide-sensing mechanism in the STC-1 cells on hydrolyzed protein. As the ELISA exhibits sensitivity to all CCK isoforms possessing the C-terminal CCK octapeptide but varying in biological activity at the CCK 1 receptor (CCK 1R), a secondary module was added to the STC-1 cell assay. Intracellular calcium measurements were performed in CHO-CCK 1R cells. Following exposure of the STC-1 cells to the protein hydrolysates, the medium was tested on the CCK 1R assay. Released CCK was measured with higher sensitivity and lower variability than in the ELISA. Surprisingly, we found that some protein hydrolysates (soy > potato >> casein) also directly stimulated CCK 1R-expressing cells, while whey and pea protein hydrolysates were inactive. As CCK 1R is expressed in the GI tract, direct interaction of CCK 1R with dietary peptides may contribute to their satiety effects. Future experiments developing bioactive ingredients for Functional Foods for weight management could involve isolation of the active, CCK 1R-activating peptides in, for example, soy protein hydrolysates.