1. 1. a- dl-tocopheryl phosphate (TPh) added in vitro inhibits crude rat liveresterase, purified horse liveresterase, horse serum pseudo cholinesterase and ox nucleus caudatus true cholinesterase. 2. 2. The percentage inhibition at a given concentration depends largely on the amount of enzyme preparation used. 3. 3. The inhibition of pseudo cholinesterase by TPh is difficult to reverse but a certain amount of reversion can be obtained by dialysis against distilled water during 7 days and by the addition of Tween 8o. 4. 4. The inhibition of pseudo cholinesterase and true cholinesterase by TPh is not (exclusively) due to the removal of Ca ++ions. 5. 5. The action of TPh is identical with that of the anionic detergents Na-lauryl sulphate and Na-cetyl sulphate. 6. 6. TPh, Na-lauryl sulphate and Na-cetyl sulphate are probably bound to the pseudo cholinesterase preparation in equal amounts, whereas about 15,000 times less of physostigmine is bound. 7. 7. a- dl-tocopherol added as an emulsion in vitro to liver homogenates of normal and E-deficient rats has no effect on liveresterase activity. 8. 8. It is concluded that TPh acts as an anionic detergents and that the in vitro effects bears no relation to the physiological action of vitamin E.