In previous studies from these laboratories, radioactive phosphorus was used to determine the rate of phospholipid turnover in 4 types of transplantable tumors: a mammary carcinoma, a lymphoma, a lymphosarcoma, and sarcoma 180 (1, 2 ) . Some distinctive features of the metabolism of these neoplastic tissues were described: (a) the fact that their phospholipid turnover bears a greater resemblance to that of the more active tissues, such as liver, kidney, and intestine, than to that of the less active tissues, such as muscle or brain; ( b ) the fact that, while each type of tumor displays a characteristic phospholipid activity, the rate of turnover is not uniform in all types. Thus the phospholipid activity of mammary carcinoma and of lymphosarcoma was at least twice as great as that observed in sarcoma 180 and lymphoma. The individuality of the phospholipid metabolism of each type of neoplasm was clearly established by employment of a method of tumor implantation that permitted the growth of tumors of two or three types under identical metabolic environment. Even under such conditions the characteristics of each tumor are retained. In the present investigation another phase of the metabolism of these neoplastic tissues has been investigated. Radioactive phosphorus was injected into tumor-bearing mice, and the rate of uptake of the total labeled phosphorus by the neoplastic tissues was compared with that of the normal tissues of the host. As regards one of these tumors, namely the lymphoma, it has previously been shown that it retains more labeled phosphorus for longer periods than liver and lymph nodes and that spleen and lymph nodes invaded by lymphoma cells show a higher phosphorus uptake than normal tissues such as muscle, liver, spleen, and lymph nodes (3, 4). .