Research work on the influence of filters humidity on bacterial survival in filter media was carried out. The results indicated that increase in water content up to 93% of weight stimulated bacterial growth. When filters humidity was stable, number of tested bacterial groups reached 106 - 107 cfu/cm2 and did not change much within 105 days of 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) number of bacteria dropped within the first 14 days to 102- 104 cfu/cm2 and Gram positive endospore-forming rods were found as dominants.