The effects of a mild degree of induced-hyperthyroidism or vit. B12 deficiency or both on growth rate were detd. in 81 immature male mice fed a vit. B12-deficient ration, in 2 exptl. trials of 26 or 30 days duration. Feeding .025% Protamone (iodinated casein) or 100 ug. of vit. B12/kilo of ration each increased the growth rate of the mice, and Protamone was more effective than vit. B12 in this respect. Each substance increased food intake and enhanced the efficiency of converting food into body wt. gains. When Protamone and vit. Bl2 were fed together, however, the favorable effects of either on growth were largely eliminated and efficiency of food utilization was reduced. No satisfactory explanation can be offered for this phenomenon.