The active compounds which have been isolated from plants and tested in the chemotherapy program of the National Cancer Institute since the inception of the plant program (as part of the Cancer Chemotherapy National Service Center) are listed, classified into types, and discussed in terms of their activity in experimental tumor systems. The tumor systems include the most important ones comprising the regular screen at different times and also the slow-growing tumors, B16 melanoma and Lewis lung carcinoma (new). The structure-antitumor activity relationships bring out the desirability for further investigation of certain types of compounds as possibilities for clinical trial. Notes on the current pharmacologic anc clinical status of certain compounds are also presented.