The in vitro activity of nitazoxanide alone and in combination with azithromycin and rifabutin was investigated against four clinical isolates of Cryptosporidium parvum. The susceptibility tests were performed by inoculation of the isolates on toe cell monolayers and determination of the parasite count after 48 h incubation at 37 degrees C. The culture medium was supplemented with Dulbecco's modified Eagle's medium containing serial dilutions of each agent. Antibiotic-free plates were used as controls. Experiments were performed in triplicate. Nitazoxanide showed moderate anticryptosporidial activity: it suppressed the growth of parasites by >50% at 8 mg/L. A parasite reduction of 79.8-83.9% was observed when nitazoxanide 8 mg/L was combined with azithromycin 8 mg/L and rifabutin 8 mg/L. The study suggests that nitazoxanide may be active in inhibiting C. parvum growth in vitro upon combination with azithromycin or rifabutin.