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The objective of this study was to evaluate antiproliferative activity, antioxidant capacity and tannin content in plants from semi-arid northeastern Brazil (Caatinga). For this study, we selected 14 species and we assayed the methanol extracts for antiproliferative activity against the HEp-2 (laryngeal cancer) and NCI-H292 (lung cancer) cell lines using the (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazole) (MTT) method. In addition, the antioxidant activity was evaluated with the DPPH (2,2-diphenyl-2-picrylhydrazyl) assay, and the tannin content was determined by the radial diffusion method. Plants with better antioxidant activity (expressed in a dose able to decrease the initial DPPH concentration by 50%, or IC50) and with higher levels of tannins were: Poincianella pyramidalis (42.95 +/- 1.77 mu g/mL IC50 and 8.17 +/- 0.64 tannin content), Jatropha mollissima (54.09 +/- 4.36 mu g/mL IC50 and 2.35 +/- 0.08 tannin content) and Anadenanthera colubrina (73.24 +/- 1.47 mu g/mL IC50 and 4.41 +/- 0.47 tannin content). Plants with enhanced antiproliferative activity (% living cells) were Annona muricata (24.94 +/- 0.74 in NCI-H292), Lantana camara (25.8 +/- 0.19 in NCI-H292), Handroanthus impetiginosus (41.8 +/- 0.47 in NCI-H292) and Mentzelia aspera (45.61 +/- 1.94 in HEp-2). For species with better antioxidant and antiproliferative activities, we suggest future in vitro and in vivo comparative studies with other pharmacological models, and to start a process of purification and identification of the possible molecule(s) responsible for the observed pharmacological activity. We believe that the flora of Brazilian semi-arid areas can be a valuable source of plants rich in tannins, cytotoxic compounds and antioxidant agents.