Chlamydia trachomatis is a common cause of sexually transmitted diseases. Recently it has been shown that chlamydiae are also responsible for complications to such lower genital tract infections. In this study, isolates of C. trachomatis from the fallopian tubes of patients with acute salpingitis were inoculated direct into the fallopian tubes of two, and through the cervical canal into the uterine cavity of one grivet monkey. The experimental infections resulted in a self-limited acute salpingitis in the three animals. C. trachomatis was recovered from the monkeys 2 and 3 weeks post inoculation. As found at laparotomy, the infected tubes were swollen and reddened, and there was watery exudate in the abdominal ostia. Microscopically, cellular infiltrates--mainly lymphocytes--were seen in the mucosa, muscularis and subserosa of the tubes. Serologically, a primary antibody response with an IgM to IgG conversion was found. Salpingitis did not occur in a control monkey inoculated in the tubes with a medium lacking Chlamydia. The histological changes in the fallopian tubes of the infected monkeys were reminiscent of those described as being characteristic of "gonococcal" salpingitis in man. The fulfilment of Koch's postulates in the animal model used adds to the earlier evidence that C. trachomatis is capable of causing acute salpingitis in humans.