Plasmin is an important enzyme in the process of wound healing and it has been suggested that it plays a role in the pathogenesis and healing of periodontal diseases. In this study, plasmin activity in crevicular fluid (CF) was measured in different clinical conditions and after periodontal treatment. 152 CF samples were collected with paper strips from 12 subjects. Five other subjects received a single periodontal treatment after the first sample collection and the post-treatment samples were collected 2, 5, 10, 20 and 40 days thereafter. The individual strips were placed immediately on plasminogen-free fibrin plates or in plastic vials containing sterile saline. The vials were agitated and 20 microliters of each solution was pipetted onto fibrin plates. The remaining solution was used in protein analysis. The plates were incubated for 24 hrs at 37 degrees C and the activity of plasmin determined by weighing the fibrin liquified by the samples. The variation of CF plasmin activity in different pockets within individuals was larger than between the subjects. There was a weak positive correlation between CF plasmin activity and the amount of plaque, bleeding tendency, pocket depth and bone loss by x-ray. However, plasmin concentration (units of plasmin activity per CF volume) did not correlate with clinical parameters. A weak negative correlation between plasmin activity per volume and the amount of CF was seen. Neither did CF protein concentration correlate with the clinical parameters. Periodontal treatment resulted in a dramatic decrease in plasmin activity. In 20 days the activity returned to pre-operative levels in four subjects out of five.