Caenopore-5 (Cp-5)1 and ameobapore A (APA)2 are members of the lipid binding saposin-like-protein (SAPLIP) family3. Similar to other SAPLIP members, these antimicrobial proteins have surface exposed charged residues that are postulated to be functionally important in lipid recognition4. Both Cp-5 and APA show pH dependent activity, indicating that the protonation state of particular amino acids is crucial in modulating activity. We have found that slightly acidic pH conditions enhance membrane interaction, suggesting that while basic amino acids initiate membrane binding via interaction with negatively charged lipid headgroups, it is the protonation state of acidic amino acids that regulates the pH-dependent activity of Cp-5 and APA. NMR spectroscopy was used to measure the pKa values of the acidic amino acid side chains and histidine side chains. To accurately monitor the pH of the NMR samples during the pH titration, a modification of the pH measurement method reported by Sykes5 was used that enabled us to measure sample pH values down to a value of pH 1. The results revealed that particular acidic amino acids in close proximity to structurally conserved basic amino acids6 on the surface of Cp-5 have pKa values that match closely the pH activity maxima, and therefore likely represent a pH-trigger for membrane association/dissociation. The pKa values of histidine side chains were also found to be elevated slightly above the intrinsic value, suggesting that these residues may also regulate activity. In case of APA, a combination of elevated pKa values for two acidic residues together with the reduced pKa of one acidic residue involved in a salt bridge suggest both the dimerization and membrane binding process is pH modulated. A model of how these proteins interact with target membranes is presented.