The objectives of this study were to investigate the: (a) presence and activity of components of the "plasminogen activators/plasmin" system in dairy cows with or without endometritis; (b) variations in enzyme activity according to the degree of endometritis; and (c) associations between these enzymes and changes in endometrial histology after intrauterine antibiotic treatment. Endometrial biopsies were collected from anestrus (no palpable ovarian structures and milk progesterone <1 ng/ml) Holstein cows, 30-40 days postpartum. On the basis of a vaginoscopic examination, rectal palpation of the cervix and uterus, and endometrial histology, there were 92 cows with endometritis and 20 cows without endometritis. After biopsy collection, each cow was given an intrauterine infusion of 1.5x10(6) IU of procaine penicillin G. In cows with endometritis, genital tract examinations and biopsies were repeated 2 weeks later. Both plasminogen activators (PAs), tissue type (t-PA) and urokinase (u-PA), were immunologically identified in all uterine biopsies. Plasminogen activator activity (PAA) increased, whereas plasminogen activator inhibition (PAI) and plasmin inhibition (PI) decreased in proportion to the degree of inflammation. Two weeks after intrauterine treatment, PAA had decreased significantly in all cows that had reduced severity of endometrial inflammation and had increased significantly in all cows with increased severity of inflammation. The change in the degree of inflammation depended upon plasminogen activator activity; cows with higher PAA were more likely to improve. In conclusion, there was evidence for a role of the plasminogen activation proteolytic system in bovine endometritis.