This study compared two published methods for assessing the in-vitro drug sensitivity of Giardia duodenalis: the 3H-thymidine incorporation assay (BOREHAM et al. 1984) which radiometrically measures nucleic acid synthesis; and inhibition of adherence (MELONI et al. 1990). Giardia trophozoites were exposed to a range of concentrations of metronidazole or albendazole for 24 hours and their viability measured by both assays in order to determine the degree of correlation between the two methods of measuring viability. Due to the different modes of action of metronidazole and albendazole on Giardia, measuring the inhibition of adherence appears to be a more accurate indicator of trophozoite viability than measuring 3H-thymidine incorporation.