1. Turkey poults were fed on an isolated soya protein diet supplemented with 0, 2, 4 or 8 mg riboflavin/kg between 7 and 28 d of age. Maximum body weight was attained with the 4 and 8 mg diets, and poults fed on the supplemented diet did not survive until 21 d of age. The erythrocyte glutathione reductase activation coefficient (EGRAC) of the poults was inversely related to dietary riboflavin at all ages. At 28 d of age the values for EGRAC of poults fed on the 2, 4 and 8 mg diets were 2.25, 1.59 and 1.25 respectively; at 31 d, following a single oral dose of 10 mg riboflavin administered on day 28, the same poults had EGRAC values of 1.36, 1.17 and 1.26. 2. In a second experiment, poults were fed on diets supplemented with 0, 2, 4, 8 or 12 mg riboflavin/kg. Maximum body weight was attained with the 4, 8 and 12 mg diets. At 28 d of age the EGRAC values for the 0, 2, 4, 8 and 12 mg treatments were 2.36, 2.34, 1.59, 1.28 and 1.18 respectively. The concentration of flavin in the liver was not related to riboflavin intake, whereas total flavin in the liver was positively related, being on average 94, 206, 400, 440 and 413 micrograms flavin/liver for the 0, 2, 4, 8 and 12 mg treatments respectively. 3. It is concluded that EGRAC is a sensitive indicator of riboflavin status in the turkey poult and that an EGRAC of 1.6 indicates a marginal riboflavin status. It is also concluded that liver total flavin reflects merely the effect of riboflavin on growth and liver size.