The protective effects of some neutral amino acids against hypotonic hemolysis were examined at various pHs. At pH 5.0, 7.0 and 8.0, 50% hemolysis was induced at 200, 160 and 140 mOsM, respectively, suggesting that erythrocyte membranes became more fragile to osmotic shock with decreasing pH. All amino acids tested reduced the hypotonic hemolysis at pH 5.0, but enhanced it at pH 8.0. It is therefore likely that these amino acids controlled the osmotic fragility of the cell membranes. At pH 7.0, glycine (Gly) reduced hypotonic hemolysis with increasing concentration. Phenylalanine (Phe) also reduced hypotonic hemolysis at low concentrations, but had an incrementally opposite effect at high concentrations. It was suggested that Phe interacted with erythrocyte membranes in a similar way to amphipathic drugs. Kinetic studies demonstrated that hypotonic hemolysis occurred immediately, according to osmotic shock, and that Gly and a low concentration of Phe decreased osmotic shock. Phe at a high concentration showed fast hemolysis with a short lag-time. Gly also showed fast hemolysis after the suppression of hypotonic hemolysis. Morphological observations demonstrated that these amino acids induced exvagination, exovesiculation and then invagination. It was suggested that with exvagination, the membrane expansion decreased the osmotic fragility, but the further shape change evoked membrane hole-formation.