Non-steroidal anti-inflammatory drugs (NSAID) elicit gastric damage through inhibition of the synthesis of prostaglandins that protect gastric cells and direct effect on mucous layer. As the latter effect is not well understood, we used acid hemolysis test in a model study on the cytotoxicity of nine NSAIDs. Human erythrocytes were used as model cells after their band 3 membrane protein was inhibited with DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonate) that strongly suppressed the entry of acid into cytosole and postponed acid-induced hemolysis. These drugs did not produce measurable hemolysis in media buffered at pH 7.2. However, in acidic media (pH 3.4) they markedly reduced to a variable extent the prelytic interval (time spent by acid to accumulate overcritically in cytosole) and time for 50% hemolysis (acid resistance). The cytotoxicity of NSAID to erythrocytes at acidic medium was expressed by the inverse of the concentration (C50%) that reduced twofold acid resistance. It was related to the hydrophobicity of drug as the log of C50% depended linearly on the log of its critical concentration for the formation of micelles. Hence, the cytotoxicity of NSAIDs to model cells in acidic media apparently involved the transfer of protonated forms and accumulation of the drug and acid into cytosole. We conclude, the protonophore mechanism could be involved in the direct damage of erythrocytes in acidic media. Based on this cytotoxicity the NSAIDs were ranked as aspirin