Acetylation of erythrocytic membranes by aspirin has not been reported. When erythrocytes or erythrocytic membranes are incubated with [acetyl‐14C]aspirin, there is good evidence of very tight binding to membrane peptides in concentrations known to occurin vivo, 20 to 200 µM. No such evidence is observed with [carboxy‐14C] aspirin. This indicates acetylation of membrane peptides. Although pronounced selectivity is not observed among the different peptides by gel filtration on Bio‐Gel A‐5M and sodium dodecyl sulfate polyacrylamide gel electrophoresis, there is overall saturation with a half‐maximum at about 10 mM. The clinical significance of acetylation of membrane peptides of erythrocytes