Research on the influence of filter humidity on bacterial survival in filter media was carried out. The results indicate that an increase in water content up to 93% of weight stimulated bacterial growth. When filter humidity was stable, the number of tested bacterial groups reached 10⁶ – 10⁷ cfu/cm² and did not change much within 105 days of the experiment. Strains of Pseudomonas dominated – their percentage in humidity 93% of weight was 92,3%. If water content in filters was not sufficient (13% of weight), the number of bacteria dropped within the first 14 days to 10²- 10⁴ cfu/cm² and Gram positive endospore-forming rods were found as dominants.