Inhibition of Transplantable Mouse Tumor Growth by Tissue Extracts and Their Protein Fractions

1. Mammary tissue of pregnant rabbits is found to contain an agent which inhibits the growth of Bashford Adenocarcinoma 63 in mice but this material is without effect on the Crocker Sarcoma 180. 2. Of the tissues so far studied the mammary gland of rabbits has yielded the most active product. 3. From the results it is probable that pregnancy enhances the production of the agent but this cannot be considered as established beyond doubt. 4. The factor is found, with diminished activity, only in the protein fraction obtained from the aqueous extract by full saturation with ammonium sulfate.