The livers of mice infected with S. mansoni showed a reduced ability to oxidize succinic acid despite the fact that endogenous respiration was little affected. The effect on the succinoxidase activity is possibly, in part, attributable to the incorporation within the liver of extra-hepatic tissues, but the existence of a toxic influence by the infection was not ignored. The total nitrogen of infected liver was slightly higher than that of normal liver and an increase in liver collagen accompanied the present infections. No effect of preliminary extracts of infected liver and adult worm were obtained on the succinoxidase system of normal mouse liver. Tyrosinase activity of mouse liver was unaffected by the infection but the glycolytic mechanism was somewhat reduced in activity.