Sprague-Dawley rats infected with Mycoplasma arthritidis by tail vein inoculation develop extensive disseminated joint inflammation, frequently accompanied by conjunctivitis and anterior uveitis. The intraocular inflammation is apparently directed at mycoplasmas localized within the stroma of the ciliary body, which have been detected with monoclonal antibodies by indirect immunofluorescence. The monoclonal antibodies are directed against an antigenic determinant on the enzyme arginine deiminase isolated from M. arthritidis, but they do not react with the same enzyme derived from Mycoplasma hominis. The antigen bound by the monoclonal antibodies can also be detected by immunofluorescence in M. arthritidis-infected tissue cultures and is not lost after glutaraldehyde fixation or paraffin-embedding procedures. The value in the application of monoclonal antibodies reactive with arginine deiminase lies in the fact that although this enzyme may be found in mycoplasmas and several other species of bacteria it is not a normal constituent of mammalian tissues.