To determine the capacity of extracts of Pelargonium reniforme and Pelargonium sidoides, plants of the Geraniaceae family, to stimulate the uptake and killing of mycobacteria by murine macrophages and to identify the constituents that are responsible. Bioassay-guided fractionation of aqueous P. reniforme extracts yielded five chemically distinct structures with the capacity to increase the rate of intracellular killing by macrophages. These were: gallic acid, methyl gallate, myricetin and quercitin-3-O-beta-d-glucoside, in addition to the previously unrecognized constituent 1-O-(2-(4-methoxyphenyl)ethyl-6-O-galloyl-glucopyranoside. Kinetics of intracellular accumulation of Mycobacterium tuberculosis and Mycobacterium fortuitum by macrophages were indistinguishable; pure preparations of the four previously known plant constituents stimulated macrophage killing, but not uptake, of M. tuberculosis and M. fortuitum equally well. A number of distinct molecular species are present in the medicinal plant P. reniforme that stimulate the killing of the intracellular pathogen M. tuberculosis. These observations support the view that Pelargonium extracts may have utility in the treatment of tuberculosis.