Piscidins 1 and 3 (P1 and P3) are potent antimicrobial peptides isolated from striped bass. Their mechanism of action involves formation of amphipathic alpha-helices on contact with phospholipids and destabilization of the microbial cytoplasmic membrane. The peptides are active against both Gram-positive and Gram-negative bacteria, suggesting easy passage across the outer membrane. Here, we performed a comparative study of these two piscidins at the air-water interface on lipopolysaccharide (LPS) monolayers modeling the outer bacterial surface of Gram-negative organisms and on phospholipid monolayers, which mimic the inner membrane. The results show that P1 and P3 are highly surface active (log K-AW similar to 6.8) and have similar affinities to phospholipid monolayers (log K-lip approximate to 7.7). P1, which is more potent against Gram negatives, exhibits a much stronger partitioning into LPS monolayers (log K-LPS = 8.3). Pressure-area isotherms indicate that under increasing lateral pressures, inserted P1 repartitions from phospholipid monolayers back to the subphase or to a more shallow position with in-plane areas of similar to 170 angstrom(2) per peptide, corresponding to fully folded amphipathic alpha-helices. In contrast, peptide expulsion from LPS occurs with areas of similar to 35 angstrom(2), suggesting that the peptides may not form the similarly oriented, rigid secondary structures when they avidly intercalate between LPS molecules. Patch-clamp experiments on Escherichia coli spheroplasts show that when P1 and P3 reach the outer surface of the bacterial cytoplasmic membrane, they produce fluctuating conductive structures at voltages above 80 mV. The data suggests that the strong activity of these piscidins against Gram-negative bacteria begins with the preferential accumulation of peptides in the outer LPS layer followed by penetration into the periplasm, where they form stable amphipathic alpha-helices upon contact with phospholipids and attack the energized inner membrane.