The present invention relates to anti-microbial compositions that may advantageously be added to vase water of cut flowers in order to prevent microbial growth, especially in vase water containing added cut flower nutrients and/or water uptake stimulants. One aspect of the invention relates to the use of an EC 1.1.3 oxidoreductase as an antimicrobial additive for vase water of cut flowers. Another aspect of the invention concerns a method of putting cut flowers into vase water, said method comprising immersing the stems of one or more cut flowers into vase water and adding an antimicrobial composition containing an EC 1.1.3 oxidoreductase to the vase water before, after or at the same time as the cut flowers are immersed into the vase water.