The vitamin sources and requirements of tapeworms present an interesting problem not only from the parasitological point of view but also from the standpoint of the physiology of the host. The possible absorption of vitamins from the mucosa of the intestine may account for some of the pathogenic effects of parasitism by tapeworms, and there is an additional point of interest in an apparent relation between the vitamins and the sex hormones, as pointed out by the work of Chandler (1943). Very little work has been done on the vitamin requirement of the cestodes. Hager (1941) found that there was a marked decrease in the egg production of Hymenolepis diminuta in rats when the hosts were kept on a milk diet and when soybean meal was substituted for yeast. A diet deficient in both vitamin B, and the G complex caused a very marked decrease in the egg output, with loss of the worms in some cases. There was no significant decrease in the egg output when only B, was eliminated from the diet, but the lack of the G complex alone did cause a decrease. Hager suggested that there is a factor in yeast associated with the G complex which is necessary for normal egg production by the rat tapeworm, but this factor is not necessarily a member of the G complex. Chandler (1943), working with Hymenolepis diminuta in rats, studied the effect of variations in the diet of the host on the worm as determined by counting and measuring the worms after autopsy. He found that this worm is totally independent of protein in the diet of the host and drew the conclusion that the worms absorbed the nitrogenous substances directly from the mucous membrane. He suggested that this dependence of the tapeworm on the mucosa of the host for nitrogenous materials might explain the cause of the stunting effect of crowding, which has been repeatedly reported. He also found that the lack of vitamins in the diet, with or without the provision of proteins, had a marked effect on the establishment of the worms in female rats, while in male rats there was no apparent effect in either case. Also, there was a marked stunting in the growth of the worms in female rats on a vitamin-deficient diet with provision of proteins, but in female rats which lacked both vitamins and proteins the worms grew to the normal size. In male rats on a vitamin-deficient diet, with or without the provision of proteins, there was no effect on the growth of the worms. In further experiments, Chandler showed that this worm is dependent on some factor contained in autoclaved yeast in the diet of the host, but the lack of the fatsoluble vitamins A, D, and E and vitamin B, had no effect on the growth of the worms in female rats. Chandler suggested that there must be some intricate involvement of the female sex hormones with protein metabolism and vitamin utilization, which as yet has not been explained. He also expressed the view that tapeworm toxicity may be due to the absorption of proteins and vitamins, and possibly hormones or other substances, from the mucosa of the intestine of the host by the worms, instead of the absorption of toxic products of the worms by the host.