Of 20,000 plant extracts submitted to the National Cancer Institute antitumor screens from 1960 to 1980, over 95% exhibited inadequate tumor-inhibiting activity or promoted tumor growth. Of these, 50 extracts representing 42 species showed significant levels of tumor growth enhancement compared to controls on the basis of tumor weights when the extracts were administered to test and control animals dosed equally on the basis of implanted tumor weights.

Because of the continuing threat of environmentally induced or promoted cancer in the human population, the species identified in this report are deemed worthy of studies designed to quantitate the risks of contact by botanists, hikers, plant hobbyists, and field workers. Even more fundamental, studies of these plants could provide knowledge of new compounds under the influence of which tumor growth is enhanced. Further studies might also reveal the mechanisms of this tumor enhancement.

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This work was supported by Contract Fund NCI-NO1-CM-33750 from the National Cancer Institute. Additional support was provided by the Elsa U. Pardee Foundation, the Arizona Division of the American Cancer Society, and the University of Arizona Alumni Fund.

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