Administration of 9α-fluoroprednisolone to mice bearing the corticoid-sensitive strain of Lymphosarcoma P1798 resulted in a prompt regression of tumor size. This regression was accompanied by a selective decrease in tumor RNA. Tumor DNA concentration remained at control levels. These changes were preceded by an increased tumor ribonuclease activity in homogenates of tumor after the in vivo administration of corticosteroids. Regression of tumor size and activation of tumor ribonuclease were not observed in the resistant strain of tumor after steroid administration.

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These studies were supported by Contract No. SA-43-ph-2388 from the Cancer Chemotherapy National Service Center, Research Grant No. A-5711 from The National Institutes of Health, and an Institutional Grant from the American Cancer Society.

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