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The newly recognized pathogenic virus-like infectious agent (VLIA), originally reported in patients with AIDS but also known to be pathogenic in previously healthy non-AIDS patients and in non-human primates, was cultured in cell-free conditions using a modified SP-4 medium and classified as a member of the order Mycoplasmatales, class Mollicutes. The infectious microorganism is tentatively referred to as Mycoplasma incognitus. M. incognitus has the unique biochemical properties of utilizing glucose both aerobically and anaerobically, as well as having the ability to metabolize arginine. Among all known human mycoplasmas, these specific biochemical characteristics were found previously only in a rarely isolated species, M. fermentans. In comparison with M. fermentans, M. incognitus appears to be even more fastidious in cultivation requirements and fails to grow in all tested mycoplasma media other than modified SP-4 medium. In addition, M. incognitus grows much more slowly, has a smaller spherical particle size and occasional filamentous morphology, and forms only irregular and very small colonies with diffuse edges on agar plates. Antigenic analysis using polyclonal and monoclonal antibodies and DNA analysis of sequence homology and restriction enzyme mappings in M. incognitus, M. orale, M. hyorhinis, M. hominis, M. pneumoniae, M. fermentans, M. arginini, M. genitalium, M. salivarium, Ureaplasma urealyticum, and Acholeplasma laidlawii revealed that M. incognitus is distinct from other mycoplasmas, but is most closely related to M. fermentans.