Gut and blood samples from 119 finishing pigs derived from 11 farms were collected during routine slaughter at an abattoir. Sections of formalin-fixed, paraffin-embedded tissues were labeled immunohistochemically using genus-specific, mouse monoclonal antibody against chlamydial lipopolysaccharide; goat polyclonal antiserum against the major outer membrane protein of Chlamydia trachomatis; and mouse monoclonal antibody against the ovine abortion subtype of C. psittaci. Gut samples from 33 of 111 (29.7%) individual pigs stained positive with the genus-specific monoclonal antibody, and of these 30 of 32 (93.7%) also reacted with the C. trachomatis-specific antiserum. Labeled inclusions were restricted to mature enterocytes of the large intestine in 33 of 111 cases. Infection of small intestinal enterocytes was noted in only one of 82 ileal samples. The blood samples were tested for antichlamydial antibodies by enzyme-linked immunosorbent assay (ELISA) and complement fixation test (CFT). With ELISA, 95 of the 115 sera tested (82.6%) yielded positive antichlamydial reactions. With CFT, 34 of the 119 sera tested (28.6%) were unequivocally positive (> or = 1:10, 100% binding), and 10 (7.6%) yielded doubtful positive reactions (1:10, 50-75% binding). Positive ELISA and CFT titers showed poor agreement (kappa = 0.112), whereas the agreement between positive findings by immunohistochemical labeling and CFT was fair (kappa = 0.205).