The effects of haemodialysate and its three peptide fractions on acetylcholinesterase (AChE, E.C. 3.1.1.7) activity in erythrocytes from healthy subjects and patients with terminal renal insufficiency treated by repeated haemodialyses have been studied. It was shown that erythrocytes from haemodialysed patients display an increased activity of the enzyme if compared with those from healthy subjects. Neither haemodialysate nor any of its three peptide fractions, when incubated with erythrocytes from healthy subjects and from haemodialysed patients, have altered the activity of the enzyme, except for fraction III at the highest concentration. This fraction exerted an inhibitory effect on AChE activity of the erythrocyte derived from healthy subjects.