Ctn [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], a downsized version of the snake venom cathelicidin-like peptide crotalicidin (Ctn), shows an unusually high lifespan (t 1/2 , approximately 12 h) in human serum, which significantly adds to its promise as an antimicrobial and antitumor agent. Herein we investigate the role of Ctn[15-34] structure on serum survival. Using a set of analogs, we show that C-terminal amidation, as well as the specific layout of the Ctn[15-34] sequence-a helical N-terminal domain followed by a hydrophobic domain-is crucial for slow degradation, and any change in their arrangement results in significantly lower t 1/2 . Aside from the privileged primary structure, features such as self-aggregation can be ruled out as causes for the long serum life. Instead, studies in other protease-rich fluids suggest a key role for certain human serum components. Finally, we demonstrate that Ctn [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] is able to induce bacterial death even after 12-hour pre-incubation in serum, in agreement with the proteolytic data. Altogether, the results shed light on the uncommon stability of Ctn [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] in human serum and confirm its potential as an antiinfective lead.