A simple and widely applicable method for preparing homogeneous and stable quality control samples in water microbiology.

Test strains suspended in skim milk, quickly frozen in dry ice-ethanol, and stored at - 70 degrees C can be used as quality control samples that are immediately available by quickly thawing at 37 degrees C. The samples remain homogeneous and stable for at least 1 year, except for Aeromonas hydrophila, which decreases 20 to 30% in 1 year.