The invention provides the use of a compound of formula (I), or metallatedderivative thereof, for killing,inhibiting or preventing the growth of a microbial biofilm: wherein X1, X2,X3, X4, Y1, Y2, Y3, Y4 and Z have meanings given in thedescription. The biofilm may be on a living or inert support. Preferably, themicroorganisms are selected from the group consistingof bacteria and fungi.