The viability of Acanthamoeba cysts after they have been stored in water at 4 degrees C for a period of 24 years was determined and to estimate their present degree of virulence as compared to its primary values. The viability of 17 amoebae isolates was examined using the eosin exclusion and in vitro excystation on agar plates (NNE). After the period of 24 years, only 3 isolates were found dead. The remaining 14 isolates after inoculation on NNE gave rise to new subcultures, although the percentage of living cysts measured by eosin exclusion was low (0-5%). Separate groups of mice were inoculated intranasally with 10 subcultures characterised by varying primary virulence. It was found that in 8 groups the mice were invaded (at varying degree), and some of them died. Taking into consideration the fact that some of the examined isolates completely lost their virulence only after 8 years of the in vitro cultivation, the obtained results are very intriguing. On the ground of these results one can assume that in the natural environment the period of viability for a cyst may be not shorter than 25 years, and, which is even more essential, they can maintain their invasive properties.