Giardia lamblia is the most common human intestinal protozoan parasite reported in the United States and England. The objective of the study was to develop a practical, reliable, rapid, microscopically-read method, comparable to excystation for determining the viability of Giardia cysts. At first an attempt was made to use methods dependent on cyst metabolism such as the activity of pyruvate: ferredoxin oxidoreductase (PFOR) and cell respiration. Though they gave promising results with trophozoites, they proved unreliable with cysts. Hence, used was a fluorescent dye exclusion technique previously proved to be successful for measuring viability in mammalian cells.