1. An increase in the liver mitotic index and a relationship between tumor size and the liver mitotic index was observed in strains C3H/He and C3Hf/He mice bearing transplanted mammary carcinoma C3HBA.

  2. A significant increase in the host liver mitotic index was also observed 48 hours after the injection of 0.4 ml. of C3HBA mammary tumor brei. The number of mitotic figures in the host liver increased as the concentration of tumor brei injected was increased.

  3. A saline extract of C3HBA mammary tumor brei also resulted in a significant increase in the host liver mitotic index at 48 hours. No increase in the host liver mitotic index was observed following the injection of the nuclear, nucleoprotein, mitochondria, and microsome cell fractions obtained from C3HBA mammary tumor tissue.

  4. The subcutaneous injection of 0.4 ml. normal C3H/He and C3Hf/He tissue brei (liver, kidney, embryo) into normal strain C3H/He and C3Hf/He mice did not result in an increase in the host liver mitotic index 1 through 3 days after injection.

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