Self-assembling into aggregates with defined structures is a common phenomenon for many peptides at high concentrations. In this study, we found that when PTP-7b (FLGALFKALSHLL), a concentration-dependent self-assembling peptide, bound to tissue cells and accumulated on cell surfaces, it migrated and self-assembled into exosome-like aggregates at certain locations on the cell membranes. Studies using confocal microscopy and scanning electron microscopy revealed that peptide PTP-7b induced cell tissue damage through a new cell lysis mechanism that involved peptide self-assembly on cell surfaces, extracting lipids from cell membranes, and transporting peptides into the cytoplasm. Peptide self-assembly attributed greatly to peptide-cell interactions and thus the biological activity of a peptide. Because peptide self-assembly was a slow process, PTP-7b-induced cell lysis showed a biphasic behavior: a gradual viability decrease was followed by a rapid decline. These results suggest that peptide self-assembly could be equally as important as charge and secondary structure of a peptide in determining the anticancer and antibacterial activities of therapeutic peptides.