Techniques are described for the isolation, microculture and assessment of viability of Giardia intestinalis. The susceptibility of five recent Brisbane isolates and the Portland 1 stock to metronidazole, tinidazole, furazolidone and quinacrine has been determined. All stocks showed little variation in sensitivity to these drugs. The 3H-thymidine incorporation assay proved to be a much more sensitive method of assessing relative drug activity than either motility or dye exclusion methods. Tinidazole proved to be the most effective drug in vitro followed by furazolidone and metronidazole. Quinacrine was the least effective. The generation time of stocks in vitro varied from 12.5 to 44.2 h and the possible significance of this to failures in treatment is discussed.