The drug sensitivities of eleven Trypanosoma evansi isolates from China were examined using two different in vitro assays, a 3H-hypoxanthine incorporation assay and a long incubation low inoculation test (LILIT). Better discrimination of the drug susceptibility of the strains was observed with the LILIT. The drug responses of all the isolates to the arsenical melarsoprol were very similar. In contrast, for suramin, minimal inhibitory concentrations (MIC) varied within a 27-fold range and for diminazene within a 55-fold range. Comparison of MIC values with expected drug levels in the host as well as in vivo experiments with selected isolates and drugs indicated that all the isolates examined would be sensitive to melarsoprol, diminazene and suramin under in vivo conditions. For isometamidium, the difference in MIC values between the most and the least sensitive isolate was 724-fold. Neither of two isolates tested in mice--the most resistant and the second most sensitive--was cured with the highest acceptable dose of 10 mg kg-1 isometamidium chloride. Comparison of our results with blood levels of drug to be expected in cattle support the assumption that the Chinese T. evansi isolates have more or less innate resistance to isometamidium under in vivo conditions. One Trypanosoma equiperdum isolate was tested in the 3H-hypoxanthine incorporation assay. The results indicated that this isolate was highly sensitive to melarsoprol, isometamidium and suramin; with regard to diminazene, T. equiperdum was not as sensitive as the most sensitive T. evansi strains.