Changes in the plasma lactic dehydrogenase (LDH) activity in mice bearing a solid mammary carcinoma or the Ehrlich-Landschütz hyperdiploid (ELD) ascites carcinoma have been studied. The Riley agent was associated with both tumors; therefore, an increase of 5–10 times the normal activity level of LDH occurred shortly after tumor inoculation. The sources of the larger increase of plasma LDH which appeared during later stages of tumor growth are discussed. The main emphasis was on the enzyme concentration gradients between the interstitial fluid compartments concerned. It was concluded that a large proportion of the increased LDH activity was released from the tumor cells. With reference to the anemia, it was shown that the contents of the destroyed red cells was only large enough to produce a small part of the observed LDH elevation.

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This work was supported by institutional grants from the Swedish Cancer Society, the Jubilee Fund, and the Jane Coffin Childs Memorial Fund, which are gratefully acknowledged. The authors are also indebted to Dr. Vernon Riley of the Sloan-Kettering Memorial Cancer Center for his discussions.

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