The fresh beef liver has been considered to be an excellent diet for rainbow trout (Salmo irideus) fry. Although several investigations have been published on the effect of liver on chicks or rats, very few publications appeared concerning the growth stimulating activities present in the fresh beef liver for fishes. This investigation was attempted to elucidate the growth stimulating substances for rainbow trout fry. The fresh beef liver was fractionated into three fractions with sucrose solution and each fractions were analyzed on amino acids and vitamins (Fig. 1 and Tables 3 and 4). To evaluate the role of vitamins, Halvers vitamin mixture, each fractions and three kinds of vitamins mixture prepared on the basis of analy-tical data (Table 4) to imitate the corresponding fractions were incorporated into the basal diet (Table 1), and fed to the rainbow trout fry, as shown in Tables 6-8. The growth response of fish suggests that the vitamins examined here may not be the growth stimulating substances in the fresh beef liver.