Proteus mirabilis strains associated with human urinary tract infections have previously been shown to secrete an extracellular metalloproteinase which cleaves serum immunoglobulin A (IgA). The enzyme has now been purified to apparent homogeneity from culture supernatants of P. mirabilis 64676. The protease activity is associated with a 50-kilodalton (kDa) protein. Unlike that of the classic IgA1 proteases, the substrate specificity of the P. mirabilis protease has been found to extend to both sublcasses of IgA, IgG, and the nonimmunoglobulin substrates, secretory component and casein. Cleavage of IgA1 and IgA2 by the P. mirabilis protease yielded fragments on sodium dodecyl sulfate-polyacrylamide gel electrophoresis whose sizes were consistent with a cleavage site outside the hinge region. Both secretory IgA1 and IgA2 were also cleaved by P. mirabilis protease, although the secretory IgA2 molecule was less readily cleaved than secretory IgA1. Free and IgA-bound secretory components were degraded to some extent by P. mirabilis protease. Cleavage of IgG, however, occurred at the hinge region as a two-stage process. The first stage was pepsinlike and generated an F(ab')2 fragment of 120 kDa and a small pFc fragment detected on nonreduced polyacrylamide gels. In the second stage, the F(ab')2 product was cleaved to yield papainlike Fab and Fc fragments, visualized as a diffuse band of 40 to 50 kDa.