Ureaplasma urealyticum strains of all serotypes express a specific human immunoglobulin A1 protease that cleaves immunoglobulin A1 to produce intact Fab and Fc fragments. The use of a variety of inhibitors suggests that the enzyme is a serine protease. N-terminal sequencing of the Fc digestion product showed that the enzyme cleaves between the proline and threonine residues 235 and 236 in the hinge region of the heavy chain of immunoglobulin A1.