I . Sheep fed on a maintenance ration of wheaten-hay chaff or of wheaten-hay chafflucerne-hay chaff ( I : I , w/w) became deficient or incipiently deficient in vitamin E.
2.Degenerative changes were observed in bone marrow and muscle, and liver function was impaired in some animals. These abnormalities were not influenced by the vitamin BIz status of the animals or by a shortage of cobalt in the rumen.3. Plasma ascorbic acid levels may not have been optimum, and folk acid may not have been fully utilized by some sheep.4. Liver function responded fairly rapidly to a-tocopheryl acetate, but skeletal muscle had not returned to normal after 28 weeks of treatment. A variable trend towards normal cellularity was found in bone marrow following supplementation with a-tocopheryl acetate. 5 . A secondary deficiency or, alternatively, inefficient excretion or metabolism of a toxic material, may occur in vitamin E deficiency as a result of degenerative changes in the absorptive or excretory areas of the intestinal tract and be responsible for the bone marrow abnormality.Hypoplasia of the bone marrow of sheep given a ration of wheaten-hay chaff deficient in cobalt was reported by Ibbotson, Allen & Gurney (1970). The abnormality was not reversed by treatment of the animals with vitamin B,, intramuscularly or with Co intravenously, but had been found only in animals in which the stores of vitamin B,, had at some time been depleted. The present study was designed to determine whether a hypoplasia of the bone marrow similar to that reported by Ibbotson et al. (1970) is reversible, and whether the abnormality is influenced by depletion of the animal's reserves of vitamin B,,, or by the presence of adequate concentrations of Co in the rumen. The intention was to test the reversibility of the condition by substituting 50% of a maintenance wheaten-hay-chaff diet with lucerne-hay chaff; this diet, with the addition of small quantities of retinol and cholecalciferol, is usually thought to supply all the nutrients required to keep an adult sheep in good health. However, during the course of the experiment it became apparent that some of the sheep were suffering from the effects of vitamin E deficiency. Treatment with this vitamin, therefore, became the test of reversibility of the bone marrow abnormality.