The leishmanicidal activity of 15 extracts and 4 pure metabolites obtained from Urechites andrieuxii, Colubrina greggii, Dorstenia contrajerva, and Tridax procumbens was evaluated using the newly developed MTS ({3-(4,5-dimethylthiazol- 2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt) assay, optimized for promastigotes of Leishmania major, Leishmania tropica, and Leishmania aethiopica, as well as for L. aethiopica axenic amastigotes. The assay was then used for calculating the percentage of viable stationary phase parasites after a 24-hr treatment with each plant extract or pure metabolite. The 3 most active samples, 2 from C. greggii (NCG-5C and DCG-3A) and 1 from T. procumbens (TPZ-2A), showed LD₅₀ values of 62.4, 7.2, and 18.5 μg/ml, respectively, on stationary promastigotes, and of 94.2, 27.1, and 95.2 μg/ml, on amastigotes of L. aethiopica. Moreover, TPZ-2A and DCG-3A significantly reduced the percentage of infected monocyte-derived macrophages (THP-1). The percentage of infected cells decreased from 69.9% ± 2.5% to 20.8% ± 2% when the cells were treated with the DCG-3A fraction and to 14.9% ± 0.5% when treated with TPZ-2A, without significantly decreasing the number of human cells. These findings indicate the presence of potentially bioactive metabolites in the roots of C. greggii and in T. procumbens and reflect the importance of pursuing the bioassay-guided purification of these metabolites.