These efforts cover both basic and applied aspects of mycoplasmas and related wall-free prokaryotes (mollicutes), including their molecular biology, membrane and cellular components involved in attachment, virulence, or immunological interrelationships, and their possible role in human diseases or diseases of uncertain etiology. Current work is directed to further characterization of a newly discovered mycoplasma (Mycoplasma genitalium) in the urogenital tract of patients with nongonococcal urethritis. The organisms share some partial antigenic relationship to another pathogenic mycoplasma (M. pneumoniae), and this relationship was thought to be mediated by similarities in surface components on the unique terminal attachment structure found in both organisms. Recently, we have shown that P1 attachment protein present in virulent strains of M. pneumoniae is not present in virulent M. genitalium strains or in other mycoplasmas with known attachment organelles (M. gallisepticum), although a number of cross-reactive but distinct proteins were apparent between M. genitalium and M. pneumoniae strains. This information indicates, for the first time, that more than one type of attachment moiety is involved in the surface parasitism by mycoplasmas and suggests that these cross-reactive proteins are responsible for the observed serological interactions between these two mycoplasmas. Additional experimental pathogenicity studies in female chimpanzees provided further evidence for the virulence of M. genitalium. Four animals challenged intravaginally with the organism shed large numbers of mycoplasma from the vagina for 14 weeks. These observations in combination with earlier studies on urethral infections induced by M. genitalium in male chimpanzees, and our failure to infect a variety of male monkeys with this organism, supports the value of the chimpanzee model to study some sexually transmitted diseases of man.