The action of tentoxin and dihydrotentoxin on human erythrocytes was studied. It was found that both toxins increased membrane lipid fluidity as indicated by spin label fatty acid spectra. In contrast, tentoxin and dihydrotentoxin caused strongly expressed conformational changes of erythrocyte membrane proteins. Specific action of both toxins was found at the concentration of 0.01 mmol/l. Small changes of membrane lipid fluidity may be the result of lipid-protein interactions. Both toxins easily penetrate erythrocyte membrane and react with cytoplasmic proteins.