Putrescine uptake and accumulation by Evernia prunastri thallus was studied. Putrescine is effectively taken by lichen cells in the mono-protonated form and then accumulated whithout significant enzymatic degradation of diamine. However, the diprotonated form, which is produced at pH 5.0, is trapped on the cell walls and a high proportion of it can be degraded before uptake by a secretable diamine oxidase, which is produced at acidic pH values.