The growth of a tumor requires the coordinated participation of many components, the tumor cells, the surrounding interstitial tissue, and the terminal structures of the blood vascular system. The term proliferation usually has a more restricted meaning, referring to the increasing number of tumor cells that result from a sequence of successive cell cycles. I propose that the term propagation be used to encompass the diverse local intercellular reactions of both normal and tumor cells that are essential for continued proliferation of tumor cells in the intact host. Most efforts at chemotherapy involve the direct action of drugs on cell proliferation by modifying specific synthetic events in the cell cycle. Our understanding of tumor biology has reached the point where we can now broaden our approach to therapy. We should consider as appropriate targets for chemotherapy the various phenomena of propagation with the expectation that we may be able to inhibit tumor cell proliferation indirectly.

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The recent investigations, of the author, described in this paper were supported by Research Grant P-442A from the American Cancer Society, Inc., by Research Grant CA-10412 from the National Cancer Institute, NIH, and by Grant DRG 950A from the Damon Runyon Memorial Fund for Cancer Research, Inc.

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