The changes in the structure and properties of erythrocyte membranes that are induced by free fatty acids and their derivatives have been studied. The state of the membrane has been evaluated using the activity of membrane-bound acetylcholinesterase (AChE), the pyrene monomer/excimer fluorescence intensity ratio as an indicator of membrane lipid microviscosity and the fluorescence of membrane-bound 1-anilinonaphtalene-8-sulphonic acid (ANS). Free fatty acids and corresponding aliphatic aldehydes induced an inhibition of membrane-bound AChE, effectively decreased the bulk lipid and protein-bound lipid microviscosity, and quenched the fluorescence of membrane-bound ANS. The type and efficiency of the enzyme inhibition, as well as the efficiency of microviscosity decrease and ANS fluorescence quenching, depended on the hydrophobicity and the end group in the effector molecule. Therefore, it is proposed that fatty acids and related compounds perturb the lipid bilayer and disturb the protein-lipid complementarity of the human erythrocyte membrane.