1. Intrapleurally inoculated suspension of tumor cells (strains S-37, S-180, carcinoma in C3H, Cloudman's melanoma, Ak4, and C1498) induced in the pleural cavity an accumulation of exudate containing numerous tumor cells, many of them showing mitoses. The proportion of leukocytes (mainly polymorphonuclears) was significant early (4–5 days) after inoculation, but always lower than the proportion of tumor cells. In the later stage (7–8 days), these cells grew in the pleural exudate almost in pure culture. A standard dose of 1,000,000–10,000,000 cells either from mashed subcutaneous implants or from serial transfers in the peritoneal fluid was used throughout.

  2. Small doses of exudate transferred into the pleural cavity of new mice induced there an accumulation of exudate with numerous tumor cells. At least five successive transfers were carried out serially with S-37, S-180, and carcinoma.

  3. In each mouse (with the exception of those with Ak4) the growth of free tumor cells in the exudate was followed by their implantation in the mediastinal pleura. They grew through the pleura into the superior or the inferior mediastinum, but, in some cases of S-37 and even more frequently in melanoma cases, they filled out most of the inoculated (right) pleural cavity and grew through the thorax wall into subcutaneous tissue where they grew luxuriantly.

  4. It is concluded that the growth of free tumor cells in the pleural exudate, as well as their growth in the peritoneal fluid (ascites tumors), and the growth of leukemic cells in the blood and the peritoneal fluid are special examples of a general phenomenon of free tumor cell growth in body fluids.

  5. It is hoped that the data reported above may serve for chemotherapeutic and radiotherapeutic assays on free tumor cells in the pleural exudate and on solid tumors in the mediastinum or in the pleural cavity.

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This work was carried out under Contract At-(40-1)-269 with the Division of Biology and Medicine, United States Atomic Energy Commission.

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