An activation study of the human carbonic anhydrase (hCA, EC 4.2.1.1) isoforms hCA III (cytosolic) and IV (membrane-associated) with a series of natural and non-natural amino acids and aromatic/heterocyclic amines is reported. hCA III was efficiently activated by d-His, serotonin, pyridyl-alkylamines, and aminoethyl-piperazine/morpholine (K(A)s of 91nM-1.12microM), whereas the best hCA IV activators were 4-amino-phenylalanine, serotonin, and 4-(2-aminoethyl)-morpholine (K(A)s of 79nM-3.14microM). Precise steric and electronic requirements are needed to be present in the molecules of effective CA III/IV activators, in order to assure an adequate fit within the enzyme active site for the formation of the enzyme-activator complex, and for efficient proton transfer processes between the active site and the reaction medium. The activation profiles of CA III and IV are distinct from those of all other mammalian CA isoforms investigated so far for their interaction with amino acids and amines.