In 46 patients with a history of habitual abortion and in 18 women with primary unexplained infertility, samples from cervical mucus and endometrial tissue have been examined for presence of T-mycoplasma, and the results compared to those obtained among 45 control subjects. Colonization of T-mycoplasma in the cervix was found to be common in all patient groups (49 to 83 per cent), whereas colonization in the endometrium was found to be significantly more frequent among the habitual abortion patients (28 per cent) and the infertility patients (50 per cent) than among the control subjects (7 per cent). In all patients having T-mycoplasma-positive endometria the organism was also isolated from the cervix, thus indicating an ascending route for the colonization of the uterine cavity. Bacteria belonging to the normal vaginal flora were frequently co-isolated from the endometrial samples along with T-mycoplasma. The women with T-mycoplasma-positive endometria showed a high incidence of postabortion fever. Treatment with doxycycline eradicated T-mycoplasma from the cervix and the endometrium and apparently led to an improvement with respect to the outcome of the posttreatment pregnancies.