The effect of four routine methods of preservation of bacteria (in Dorset medium, under mineral oil, freeze-drying, cryoconservation in liquid nitrogen) on the biochemical characteristics of 11 E. coli strains from the collection of NBIMCC, was followed up. After 1 and 3 years of preservation, half of the strains changed their biochemical activity, 33 % of the parameters examined having turned into negative. The biochemical characteristics of the strains studied turned out to best preserved in Dorset medium and under mineral oil. Freeze-drying is appropriate for long-term preservation but it requires additional following up of the biochemical parameters.