Experimental studies have shown that a variety of chemopreventive plant components affect tumor initiation, promotion, and progression and the main difference, between botanical medicines and synthetic drugs, resides in the presence of complex metabolite mixtures shown by botanical medicine which in turn exert their action on different levels and via different mechanisms. In the present study, we performed an in vitro screening of ethanol extracts from commercial plants in order to investigate potential antitumor activity against human tumor cell lines. Experimental results obtained through a variety of methods and techniques indicated that extracts of I. verum, G. glabra, R. Frangula, and L. usitatissimum present significant reduction in in vitro tumor cell proliferation, suggesting these extracts as possible chemotherapeutical adjuvants for different cancer treatments.