In recent years, some beneficial effects on health of melanins that are formed as a result of polyphenol oxidase action have been discovered. However, it is also known that similar polymers have a detrimental effect on proteases activity. Thus, the aim of this work was to assay whether melanins from two different substrates act as carboxypeptidase A and carboxypeptidase B inhibitors. It was found that melanin synthesized from l-tyrosine decreases both the apparent affinity between carboxypeptidases and their substrates and the maximum reaction rate. Consequently, it is possible to conclude that these melanins inhibit CPA and CPB activity in a mixed way. On the contrary, no inhibition was found in the case of melanin synthesized from 4-methylcatechol.