Since the first report of Dienes and Edsall (1937), a number of communications have recorded the isolation of pleuropneumonia-like organisms (PPLO) from the genital tract of the human female, especially in association with inflammatory diseases such as bartholinitis, pelvic abscess, and salpingitis (Melen and Odeblad, 195 1; Randall, Stein, and Ayres, 1950; Dienes, Ropes, Smith, Madoff, and Bauer, 1948; Klieneberger-Nobel, 1945, 1959, 1962). In only a few cases has a search for specific antibodies been undertaken. Beveridge, Campbell, and Lind (1946) found antibody in a proportion of the human sera they examined, but were unable to correlate the occurrence of antibody with any clinical condition. Stokes (1955) detected antibody against PPLO in four patients (two with puerperal sepsis, one with pyosalpinx, and one with empyema); PPLO but no other pathogens were isolated from the blood, genital tract, or operation wound. Stokes (1959) reported a further case of infection following panhysterectomy, in which PPLO were isolated from the blood and antibody was present in high titre. Melen and Gotthardson (1955) found high titres of PPLO antibody in five patients with salpingo-oophoritis. Card (1959) examined serologically 56 PPLO strains isolated from the human genital tract and found that they belonged to one broad serological group; she distinguished them serologically from PPLO isolated from the mouth, from sewage, and from various animals. This serological type-specificity of the genital strain made possible a survey of a large number of sera in terms of antigens from a single representative PPLO strain. Sera from patients with venereal infections, non-venereal diseases, healthy blood donors, and children were examined. Female patients from venereal disease clinics had