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1. Labelled serum protein appeared in the blood of normal and Morris hepatoma 5123 TC bearing rats 14 to 15 min after intracaval injection of radioactive leucine. Within this time albumin synthesis can be studied separately in vivo in liver and hepatomas. .Tumor bearing rats were killed within I 0 to 14 min after injection of [3H] or [14C]leucine. Postmitochondrial supernatants were prepared from 1%-labelled liver and 3H-labelled tumor, treated with sodium deoxycholate and combined. Albumin was purified from the mixture. The ratio [3H]protein to [Wlprotein decreased during purification from 7 to 1.2, suggesting that the tumor synthesized less albumin compared to total protein than the liver.3. Radiochemically pure albumin was isolated from liver and tumor homogenates from rats killed within 12 to 14 min after injection of ~-[1-14C]leucine. The specific radioactivity in albumin isolated from liver was 13 times higher than in that from tumor.4. The proportion of the incorporation of ~-[ l -~~C ] l e u c i n e into albumin to that into total protein was found to be 3.3O/, for liver and 0.8O/, for hepatoma homogenates.I n hepatectomized rats we demonstrated that the Morris hepatomas 5123 TC and 9121 and the Reuber hepatoma H 35 TC secreted little or no protein into the blood [1]. Those experiments gave no explanation for the mechanism of the observed failure of secretion of serum proteins by the hepatomas. Possibly, serum proteins were synthesized, but not released by the tumor, as suggested for the hepatomas 5123 and H 35 by Essner and Novikoff [ 2 ] , who reported the accumulation of electron-dense material in cytoplasmic granules. Another explanation would be that no serum proteins a t all were made in the hepatomas.This paper gives information whether, and to what extent, the tumor 5123 TC synthesizes serum albumin. Part of the work has been published in the form of abstracts [3,4]. MATERIAL AND METHODS Animals and TumorsAnimals were inbred Buffalo rats. They were kept in air conditioned rooms of 20" with regulated lighting (light, 9 a.m.-9 p.m.; dark, 9 p.m. to 9 a.m.) [5] and fed ad libitum with Altromin R

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