There is increasing interest in the bioactivity of peptides carrying out antiproliferative, antihypertensive, antimicrobial, antioxidant, anticholesterolemic, opioid, and antidiabetic activities. The bioavailability of peptides depends on how readily they are digested by endopeptidases and their ability to pass through cell membranes, features that are determined by the peptide’s chemical and physical structure. On the basis of structures present in peptides that have biological activity, particularly antiproliferative activity, the tripeptides AcGly-Phe-Asn(OH) and AcGly-Phe-Asn(NH) have been designed and synthesized, then tested for their antiproliferative activity on human breast adenocarcinoma cells (MDA-MB 231) and human dermal fibroblasts (HuDe). The results show that the peptides significantly affect the proliferation of MDA-MB 231 and HuDe cells, with differentiated response between tumor and normal cells, and thus indicate that C-terminal amidation plays a role. Interestingly, the activity of both peptides in dermal fibroblasts follows the characteristic biphasic pattern of hormesis, a dose-response relationship.