Weanling rats were restricted to mineralized evaporated and mineralized dried whole milk diets to which succinylsulfathiazole (SST) was added in amounts up to 10%. Growth was markedly impaired at a level of 10% SST. The effect was less severe at lower concentrations. Either “folic acid” or solubilized liver concentrate caused prompt growth resumption. Xanthopterin alone, and a mixture of thiamine, riboflavin, pyridoxine, nicotinic acid, calcium pantothenate, and inositol, had no effect. A slight prolongation in the prothrombin time occurred in animals given 10% SST. This returned promptly to normal when 2-methyl-1,4-naphthoquinone was fed. Marked reduction in the concentration of coliform bacteria in the cecum occurred in animals given SST. The total bacterial count was appreciably decreased. The results are interpreted as further evidence of a low concentration of “folic acid” and vitamin K in evaporated and dried cows' milk.