In three experimental series the reliability of contemporary microbial indicators has been verified. It was shown that about 30% colonies growing on the Endomedium belongs not to family Enterobacteriaceae. The share of the so-called faecal coliforms growing at 44 degrees C is 54.6%. In comparison with the initial cultivations at the second passage this part increasing at about 126%. In waters of various quality is the viability of E. coli the smallest. Also on the Mc Conkey medium at 44 degrees C disappeared the colonies in order: E. coli (3-14 days), K. pneumoniae (3-21 days) C. freundii (21-28 days). This results brings an evidence that the existing indicator system for water quality should be revised.