The oviducts of two grivet monkeys and three marmosets, all sexually mature animals, were inoculated with Mycoplasma genitalium at laparotomy. The mycoplasma was not recovered from the grivet monkeys, nor from the oviducts of the marmosets although it was isolated intermittently from the vagina of two of the latter animals up to 4-6 weeks after inoculation. In contrast, all of the animals developed antibody to M. genitalium measured by a micro-immunofluorescence technique. It developed rapidly in the grivet monkeys but slowly in the marmosets, being detected first about 1 month after inoculation with a maximal response by 2 months. Furthermore, despite an absence of vaginal discharge or cytological response, all the animals developed a moderate to severe endosalpingitis characterized by the infiltration of acute inflammatory cells into the tubal epithelium, together with a lumenal exudate and adhesions between the mucosal folds. The changes are similar to those produced by Chlamydia trachomatis in simian models and naturally in women.