The possibility that the high frequency of treatment failures in Indian kala-azar might be due to infection with antimony-resistant strains of Leishmania donovani has not been experimentally addressed. L. donovani isolates were obtained from splenic aspiration smears of 24 patients in Bihar, India, who either did not respond (15) or did respond (9) to 1 or more full courses of treatment with sodium antimony gluconate (SAG). A strong correlation (P