A total of 14 marmosets were inoculated intra-vaginally with Chlamydia trachomatis and the development of genital tract disease was assessed microbiologically, by colposcopic examination of the cervical and vaginal mucosa and by cytological and histological examination of biopsy and autopsy specimens. Eight animals had infections which persisted microbiologically for 2-5 weeks, and six animals, three of which had been inoculated on multiple occasions in a previous study, apparently eliminated their infection within 1 week of inoculation. Colposcopic examination showed that four of the eight infected animals developed acute cervicitis characterized by erythema, occasional slight oedema, and the presence of cloudy or purulent cervical mucus. The other four infected animals showed minimal signs of cervical inflammation. Three of the six animals which rapidly cleared their infections developed slight cervical inflammation characterized by erythema and cloudy cervical mucus during the fortnight after inoculation. Six control animals inoculated with medium displayed minimal cervical changes. The diagnosis of acute cervicitis in the infected animals was confirmed by examination of histological sections and cervical smears, which revealed the presence of polymorphonuclear leucocytes, with lymphocytes occasionally being seen. Although inflammatory changes were occasionally noted in specimens from control animals, the changes were relatively mild and occurred at a later time than those seen in infected animals. Intracytoplasmic chlamydial inclusions in epithelial cells were not detected in any of the specimens studied. Examination of genital tract tissue obtained at autopsy from five inoculated animals generally showed inflammation of the cervix and vagina, but in only one of these animals was there evidence of endometriosis and salpingitis.(ABSTRACT TRUNCATED AT 250 WORDS)