Infection of ewes with 'Chlamydia psittaci' may result in abortion or weak lambs. Although ewes which have aborted are fertile and have normal offspring, chronic infection and vaginal shedding have been demonstrated in experimentally infected ewes (Papp 'et al'., 1994, Papp and Shewen 1996). Therefore, continual breeding of ewes that have previously aborted may be partially responsible for the dissemination of infection within flocks, particularly if rams are vectors. Intravaginal inoculation of 16 commercial ewes was used to evaluate infection by the mucosal route and the antibody response. Inoculation resulted in infection but did not stimulate a systemic immune response or abortion, however 2 rams which bred these ewes became infected. To determine whether persistent infection occurs under natural conditions, a flock with a history of 'C. psittaci' abortions was studied. Polymerase chain reaction (PCR) confirmed Chlamydiae within 7 of 21 reproductive tracts tested. Chlamydia specific serum IgA was more consistently associated with PCR detection and persisted longer than specific serum IgG. No chlamydial abortion was diagnosed during the study. However, ewes with elevated specific IgG produced lambs of lower viability and had fewer lambs weaned in 2 of the 3 years studied although not significantly different from others. To evaluate venereal transmission, a method was developed for experimentally infecting rams. 'C. psittad' was inoculated into the preputial cavity and organisms were detected within the semen and tissues of the reproductive tract. Specific IgG in the seminal plasma was the immunoglobulin most likely to increase in response to infection. The Clearview test for chlamydial lipopolysaccharide (LPS) on samples from the preputial epithelium, more consistently predicted infection (47.6%) than testing semen with either PCR or flourescein isothyocyanate (FITC) stain. Agreement between positive PCR tests of semen and positive Clearview tests of preputial epithelium from the same rams was 50%. Infection of the male reproductive tract is most likely identified if Clearview tests of preputial epithelium samples and PCR of semen are combined. Specific IgG in seminal plasma is likely to increase in response to intrapreputial infection and may be the oglobulin with the highest predictive value for infection.