Seaprose is a semi-alkaline proteinase produced by Aspergillus melleus. The aim of our study was to further characterize the properties of this enzyme, particularly looking at its interaction with alpha 1-proteinase inhibitor, the major human plasma proteinase inhibitor. We studied the cleavage of three synthetic peptide substrates induced by seaprose and the inhibitory profile of the enzyme by means of a panel of inhibitors, including alpha 1-proteinase inhibitor. The interaction between seaprose and alpha 1-proteinase inhibitor was also studied with SDS-PAGE. Finally, the elastolytic activity of seaprose was checked by means of bovine elastin solubilization. We found that seaprose cleaves preferentially the substrate containing a Phe residue in the P1 position. The inhibitory profile showed that seaprose is a serine-proteinase that cannot be inhibited by alpha 1-proteinase inhibitor. The SDS-PAGE revealed that alpha 1-proteinase inhibitor, after incubation with seaprose, underwent a limited proteolysis. Finally, seaprose 10(-2) M and 10(-3) M was able to solubilize bovine elastin. We conclude that seaprose is a serine-proteinase able to inactivate human alpha 1-proteinase inhibitor with limited proteolysis at (or near) the active site and that it has mild elastinolytic capacity.