Evidence was provided that in a repeatedly irradiated bacterial suspension of Escherichia coli K12: AB 1157, 1886, 2463, under the described experimental conditions, gradually mutants more resistant to UV radiation survive. Under these circumstances it may happen that selection and an increase of bacterial clones with an increased UV resistance develop, manifested by a reduced effectiveness of UV radiation during repeated irradiation. Although the cumulating damage of DNA in microbes can lead to a reduction of their survival as well as a reduction of their absolute numbers, it can be hardly assumed that during practical use in air conditioning units or during disinfection of drinking water all bacteria will be destroyed. In that case even repeated UV irradiation is not necessarily a reliable disinfection tool.