Caerin peptides are stable 5 or 10 μg of Caerin 1.1 (F1) and Caerin 1.9 (F3) peptides were added to 8 × 103 of TC-1 cells in flat bottomed 96 well plates and cultured overnight at 37 °C with 5% CO2. Cell proliferation was determined by an MTT assay. (a): MTT assay after TC-1 cells were incubated with Caerin 1.1 for 24–72 h. (b): Caerin 1.1 and 1.9 were left at room temperature for up to 7 days before processing to an overnight MTT assay. (c): MTT assay after the pH of the PBS was adjusted to acidic condition (from pH 7.4 to pH 5.5), and after the Caerin 1.1 and 1.9 was heated at 100 °C for 15 min. *:P < 0.05; **: P < 0.01; ***: P < 0.001.