SummaryDecrease of L-ascorbic acid synthesis by the liver microsomes of C57/BL mice bearing Ehrlich ascites carcinoma for 13 days could be accounted for by decreased food intake of the animals. On the other hand, the decreased ascorbic acid synthesis by liver microsomes from C3H/Crgl mice infected with Gardner lymphosarcoma was due neither to inanition nor to gross infiltration of the liver with tumor cells but to a decrease in the L-gulono dehydrogenase activity. There was no change in the synthesis of ascorbic acid by liver microsomes from C57/ BL mice made immune to Ehrlich ascites carcinoma cells, although the amount of mi-crosomal protein per gram liver was significantly increased. Neither was there a significant inhibition in the synthesis by the liver microsomes of C3H/Crgl bearing T-15 hepa-toma or of A/HC mice bearing L No. 2 lymphoma.