The first activation study of the human carbonic anhydrase (hCA, EC 4.2.1.1) isoforms associated to tumors, hCA IX and XII, with a small library of natural and non-natural amino acids as well as aromatic/heterocyclic amines is reported. hCA IX was activated efficiently by dopamine, adrenaline and heterocyclic amines possessing aminoethyl-/aminomethyl-moieties (K(A)s of 9 nM-1.07 microM), whereas the best hCA XII activators were serotonin, L-adrenaline, 4-(2-aminoethyl)-morpholine and d-Phe (K(A) of 0.24-0.41 microM). Precise steric and electronic requirements are needed to be present in the molecules of effective hCA IX/hCA XII activators, in order to assure an adequate fit within the enzyme active site cavity for the formation of the enzyme-activator complex, and for an efficient proton transfer process within this complex, leading to the release of a proton and formation of the catalytically active, zinc-hydroxide species of the enzyme. Selective activation of these CA isoforms might be useful to develop pharmacologic tools or to understand whether some of these biogenic amines/amino acids may influence the progression of tumors overexpressing CA IX and/or CA XII.