Various sorts of water (e.g. bathing water, tap water) are frequently monitored for their microbiological quality. False analytical results may have economic and public health consequences such as: unnecessary delay in taking repaired drinking water system back into use, unnecessary corrective actions after tests with false positive results, unnecessary condemnation of water sources for e.g. food industry, unnecessary constraints on the trade of mineral water or shellfish, unnecessary condemnation of bathing beaches, illness caused by undetected microbes in polluted water, etc. Intercomparisons have indicated that 5-15 % of all results are incorrect. Stability is the main difficulty in developing microbiological RM's. There is evidence that good stability can be achieved by isolating microbes in milk in gelatine capsules.