The search for antibiotics with a new mode of action led to numerous studies on antibacterial peptides. Most of the studies were carried out with L-amino acid peptides possessing amphipathic α-helix or β-sheet structures, which are known to be important for biological activities. Here we compared the effect of significantly altering the sequence of an amphipathic α-helical peptide (15 amino acids long) and its diastereomer (composed of both L- and D-amino acids) regarding their structure, function, and interaction with model membranes and intact bacteria. Interestingly, the effect of sequence alteration on biological function was similar for the L-amino acid peptides and the diastereomers, despite some differences in their structure in the membrane as revealed by attenuated total reflectance Fourier-transform infrared spectroscopy. However, whereas the all L-amino acid peptides were highly hemolytic, had low solubility, lost their activity in serum, and were fully cleaved by trypsin and proteinase K, the diastereomers were nonhemolytic and maintained full activity in serum. Furthermore, sequence alteration allowed making the diastereomers either fully, partially, or totally protected from degradation by the enzymes. Transmembrane potential depolarization experiments in model membranes and intact bacteria indicate that although the killing mechanism of the diastereomers is via membrane perturbation, it is also dependent on their ability to diffuse into the inner bacterial membrane. These data demonstrate the advantage of the diastereomers over their all L-amino acid counterparts as candidates for developing a repertoire of new target antibiotics with a potential for systemic use.