We recently demonstrated the presence of matrix metalloproteinases (MMPs)-1, -2, and -3 in AA amyloid deposits, which lead us to speculate that MMPs may participate in amyloidogenesis by either processing the precursor protein, or by degrading the amyloid deposits. Here we investigated this theory by determining the ability of MMP-1, -2, and -3 to degrade human acute-phase serum amyloid A (SAA) and human AA amyloid fibril proteins (AFPs). The following in vitro degradation experiments were performed: using either recombinant MMP-1, -2, or -3 and SAA as a substrate; using either recombinant MMP-1, -2, or -3 and AFP as a substrate; and using THP-1 cells as the protease source and AFP as the substrate. All three MMPs were able to cleave SAA and AFP within the region spanning residues 51 to 57. The following cleavage sites were identified: at 57 to 58 for MMP-1; at 7 to 8 and 51 to 52 for MMP-2; at 7 to 8, 16 to 17, 23 to 24, 51 to 52, 55 to 56, 56 to 57, and 57 to 58 for MMP-3. Cell culture experiments showed that THP-1 cells were able to degrade AFPs. Degradation was significantly delayed after addition of a general metalloproteinase inhibitor (o-phenanthroline) to dextran sulfate-stimulated cells. This is the first study to show that human SAAs and AFPs are susceptible to proteolytic cleavage by MMPs. Immunocytochemistry and electron microscopy showed that degradation takes place in the pericellular or extracellular compartment.