1. Harvest cells from cultures that are at or near peak density. Aseptically transfer the broth overlay to a plastic centrifuge tube and adjust the concentration of cells to 2 x 10⁷/ml in fresh medium (broth overlay). If necessary, cells may be concentrated by centrifugation at 800 x g for 5 min.
2. Prepare a 10% (v/v) solution of sterile DMSO in fresh medium (broth).xa0 Cool on ice.
3. Mix the cell preparation and the DMSO solution in equal portions. The final concentration will be 10⁷ cells/ml and 5% (v/v) DMSO. The time from the mixing of the cell preparation and DMSO solution to the start of the freezing process should be no less than 15 min and no longer than 30 min.
4. Dispense in 0.5 ml aliquots into 1.0 - 2.0 ml sterile plastic screw-capped cryules (special plastic vials for cryopreservation).
5. Place the vials in a controlled rate freezing unit.xa0 From room temperature cool at -1°C/min to -40°C.xa0 If the freezing unit can compensate for the heat of fusion, maintain rate atxa0xa0xa0xa0xa0xa0xa0 -1°C/min through the heat of fusion.xa0 At -40°C plunge into liquid nitrogen. Alternatively, place the vials in a Nalgene 1°C freezing apparatus.xa0 Place the apparatus at -80°C for 1.5 to 2 hours and then plunge ampules into liquid nitrogen.xa0 (The cooling rate in this apparatus is approximately -1°C/min.)xa0xa0
6. The frozen preparations should be stored in either the vapor or liquid phase of a nitrogen refrigerator. Frozen preparations stored below -130°C are stabile indefinitely. Those stored at temperatures above -130°C are progressively less stabile as the storage temperature is elevated. Vials should not be stored above -70°C.
7. To establish a culture from the frozen state place an ampule in a water bath set at 35°C. Immerse the vial just to a level just above the surface of the frozen material. Do not agitate the vial.
8. Immediately after thawing, do not leave in the water bath, aseptically remove the contents of the ampule and inoculate a 16 x 125 mm screw-capped test tube containing ATCC medium 807.
9. Incubate the culture vertically at 25°C. Observe the culture daily and transfer when numerous trophozoites are observed.

David S Lindsay, personal communication