Rosmarinic acid (RA) is a natural polyphenol contained in many aromatic plants with promising biological activities. Carbonic anhydrases (CAs, EC 220.127.116.11) are widespread and intensively studied metalloenzymes present in higher vertebrates. Acetylcholinesterase (AChE, E.C. 18.104.22.168) is intimately associated with the normal neurotransmission by catalysing the hydrolysis of acetylcholine to acetate and choline and acts in combination with butyrylcholinesterase (BChE) to remove acetylcholine from the synaptic cleft. Lactoperoxidase (LPO) is an enzyme involved in fighting pathogenic microorganisms, whereas glutathione S-transferases (GSTs) are dimeric proteins present both in prokaryotic and in eukaryotic organisms and involved in cellular detoxification mechanisms. In the present study, the inhibition effects of rosmarinic acid on tumour-associated carbonic anhydrase IX and XII isoenzymes, AChE, BChE, LPO and GST enzymes were evaluated. Rosmarinic acid inhibited these enzymes with K(i)s in the range between micromolar to picomolar. The best inhibitory effect of rosmarinic acid was observed against both AChE and BChE.