A total of 17 Leishmania isolates, 6 of them isolated from antimony-resistant patients, were collected in the Sudan and tested for their sensitivity to sodium stibogluconate (Pentostam) as promastigotes. Six of those isolates were tested as amastigotes infecting a murine macrophage cell line. The results indicated that the conventional promastigote screening assay did not correlate with the clinical picture, whereas the amastigote/macrophage system produced results that pertained to the in vivo responses to the drug. A laboratory-generated resistant strain of L. major was adapted to grow at a high concentration of Pentostam (1000 micrograms/ml) as promastigotes but was quite sensitive to the drug at much lower concentrations in the amastigote/(macrophage system (20 micrograms/ml), thus suggesting that Pentostam's inhibitory action is mediated through the macrophage rather than through a direct toxic effect exerted on the parasite.