Giardia muris cysts, isolated from mouse feces, excysted routinely at levels greater than 90%, when induced in 1X Hanks' supplemented with 17 mM glutathione, 29 mM L-cysteine-HC1, and 50 mM NaHCO3 for 30 minutes at 35 deg. C, followed by washing and suspension in trypsin-Tyrode's solution at pH 8.0. Although trypsin was not required in this final step, it enhanced the escape of the trophozoites from their cysts. Giardia muris excystation was dependent upon the length of the induction period, pH, oxidation-reduction potential, and temperature. Optimal induction conditions for excystation were: an induction period of 5 to 30 minutes; pH of 2; 120 mV oxidation-reduction potential; and a temperature around 35 deg. C. A gradual decline in excystation occurred as pH and oxidation-reduction potential were changed to 7 and 57 mV, respectively. There was a pronounced increase in excystation percentages with increasing temperatures between 0 and 37 deg. C.