The aetiological role of Chlamydia trachomatis (CT) and herpes simplex virus (HSV) was investigated in 189 patients with threatened abortion. Assessment of infection was based on isolation, and on determination of serum immunoglobulin (Ig)G and IgA antibodies as well as cervical IgA antibody levels with new sensitive radioimmunoassay (RIA) techniques. One third of the women were delivered of a healthy infant and two thirds aborted, but the two groups were otherwise clinically similar. By isolation, only 2.7% of the patients were CT‐positive, but increased cervical IgA antibody level to CT was detected in 41.3%. The mean level of these local antibodies was similar in both study groups, but the mean levels of serum IgA and IgG antibodies were somewhat higher in the patients who aborted although the difference was not significant. None of the cervical specimens was positive for HSV by isolation but the cervical IgA antibody level to HSV was raised in 47.1% of the patients. Both cervical and serum IgA antibody levels to HSV were significantly raised among the patients who aborted, but there were no differences between the patients with spontaneous abortion and those with a blighted ovum. There was no clear association between CT and abortion, but an association between HSV and abortion is possible. The incidence of raised levels of both CT and HSV IgA antibodies in the cervix was surprisingly high in both groups and the significance of this finding remains to be investigated.