Thirteen serovars of Ureaplasma urealyticum were analyzed for the ability to cleave human IgA. Strains of all of the serovars tested cleaved IgA 1 in the hinge region of the alpha-chain, resulting in intact Fc and monomeric Fab fragments. IgA 1 protease activity was also observed in concentrated cell-free extracts of spent cultivation medium, indicating that the IgA 1 protease was excreted into the medium during growth of the micro-organisms. Five clinical isolates of U. urealyticum obtained from urine, cervix, vagina, amniotic fluid, and synovial fluid were positive for IgA 1 protease activity. No proteolytic activity was observed against human IgA 2, IgG, or IgM. Strains of Mycoplasma fermentans, M. salivarium and seven serovars of M. hominis were negative for IgA 1 protease activity.