Average growth of a group of rats fed the U.S.P. XII vit. A-free diet plus 50 I.U. vit. A/day was 135.3 g. in 8 wks. Supplementary feeding of liver extract at 0.05 ml./day to another group increased the growth to 172.4 g. Replacing half of the casein in the U.S.P. diet plus vit. A by extracted pork muscle or sardine meal also increased growth to 162.0 g. and 191.8 g. respectively. This compared with 183.7 g. on stock diet. The factor probably responsible for this increased growth has been studied by other workers. Its presence in a variety of materials makes its inclusion in vit. A bio-assay diets desirable.