Abstract. Two serological tests (indirect immunofluorescence and enzyme-linked immunosorbent assay) were developed for the detection of fetal antibody to Chlamydia psittaci. Fetal blood and thoracic fluid from 126 field cases of suspected ovine chlamydial abortion were examined using both tests. Placenta and fetal tissues (lung, liver, and kidney) from the same animals were also examined by the following conventional diagnostic methods: isolation in McCoy cells, detection of chlamydial lipopolysaccharide (LPS), modified ZiehlNielsen staining, and direct fluorescent antibody staining of chlamydia in frozen cryostat sections. Seventy cases were positive by fetal serology, and of these, 68 were also positive by isolation and/or LPS detection. The remaining 56 cases had negative fetal serology, and of these, 39 were positive by isolation and/or LPS detection. Results indicate that fetal serology, although less sensitive than either isolation in McCoy cells or detection of chlamydial LPS antigen, may be of particular use when placenta is not available.Enzootic abortion of ewes (EAE), induced by Chlamydia psittaci infection, occurs in most sheep-rearing countries 31 and is a source of major economic loss. In the United Kingdom, C. psittaci is the most common cause of ovine abortion, 2 and the incidence of this disease is increasing in Northern Ireland. 20 In recent reports, abortions in dairy herds 9,14 have also been attributed to C. psittaci infection. Isolation and characterization of the abortion-causing strain from 1 herd indicated that it was serotype 1. In addition, nucleotide sequencing of the major outer membrane protein (MOMP) showed it to be identical to the known ovine abortion strain. 13 The transmission of ovine strains of C. psittaci to pregnant women, causing placentitis and abortion, has also been well documented. 5,35 Diagnosis of EAE, by examination of fetal tissues following postmortem examination, can be achieved by a variety of methods. With the exception of isolation, which is dependent on the presence of viable chlamydiae in the sample, all of these methods are based on direct antigen detection and include modified Ziehl-Nielsen (MZN) staining of placental smears 30 and direct fluorescent-antibody staining of chlamydiae in frozen cryostat sections of placenta or fetal tissues.