Abstract—Diphtheria toxin (DT) did not produce measurable degradation of myelin proteins or sulphatide in sciatic nerves of chick embryos after incubationin vitrofor 4 h. In contrast, DT inhibited thein vitroincorporation of L‐[U‐14C]leucine into myelin proteins by the nerves after a delay of 1 h. Separation of the myelin proteins by SDS‐polyacrylamide gel electrophoresis indicated that the synthesis of Wolfgram proteins and proteins not entering the gel was inhibited by 21–22 per cent, whereas synthesis of myelin proteolipid and basic proteins was inhibited by 79–88 per cent. Incorporation of35SO4into myelin [35S]sulphatide was also inhibited by DT after a delay of 2 h. The inhibition of [35S]sulpha‐tide incorporation into myelin caused by DT differed from that observed with puromycin in that it did not depend on depletion of an intracellular transport lipoprotein. Instead, the inhibition seemed to be secondary to the decreased synthesis of myelin proteolipid and b