1. Harvest cultures when cells have fully encysted, using a cell scraper or rubber policeman to detach adherent cysts.
2. Adjust the concentration to approximately 2 x 10⁵ cysts/mL by centrifugation at 1300 x g for 5 min and resuspend the pellet in the volume of fresh medium required to yield the desired concentration.
3. While cells are centrifuging prepare a 20% (v/v) solution of sterile DMSO as follows:xa0 Add the required volume of DMSO to a glass screw-capped test tube and place it in an ice bath.xa0 Allow the DMSO to solidify.xa0 Add the required volume of refrigerated medium.xa0 Dissolve the DMSO by inverting the tube several times.xa0
   Note: If the DMSO solution is not prepared on ice, an exothermic reaction will occur that may precipitate certain components of the medium.
4. Mix the cell preparation and the DMSO in equal portions. Thus, the final concentration will be approximately 10⁵ cysts/mL and 10.0% (v/v) DMSO. The time from the mixing of the cell preparation and DMSO stock solution to the start of the freezing process should be no less than 15 min and no longer than 60 min.
5. Dispense in 0.5 mL aliquots into 1.0 - 2.0 mL sterile plastic screw-capped cryules (special plastic vials for cryopreservation).
6. 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 at -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 approximatelyxa0-1°C/min.) xa0
7. The frozen preparations are stored in either the vapor or liquid phase of a nitrogen freezer.
8. To establish a culture from the frozen state, place the vial in a 35°C water bath.xa0 Immerse the vial to a level just above the surface of the frozen material. Do not agitate the vial.xa0xa0 Immediately after thawing, do not leave in water bath, aseptically remove the contents of the ampule and inoculate into a T-25 tissue culture flask containing 10 mL ATCC medium 802 bacterized with Klebsiella pneumoniae subsp. pneumoniae (ATCC^® 700831™) or Enterobacter aerogenes (ATCC^® 13048™).
9. Incubate with the cap tightly sealed at 20-25°C.
10. Once the culture is established, follow the protocol for maintenance of culture.

Little Falls runoff area, Maryland shore of Potomac River