We studied the quantitative fluctuations of tissue plasminogen activator (t-PA) activity and antigen in aqueous humor before and after extracapsular cataract extraction and poly(methyl methacrylate) posterior chamber lens implantation. The t-PA activity level was measured by solid phase bioimmunoassay using monoclonal antibody against an epitope apart from the active site of t-PA, and the antigen by ELISA. In our patients the mean preoperative level of t-PA activity was 0.0664 +/- 0.0472 IU/ml (mean +/- SD) and of the antigen, 0.175 +/- 0.024 ng/ml. The t-PA activity level in aqueous humor was markedly decreased on the first postoperative day (0.0042 +/- 0.0037 IU/ml), recovered on the second day (0.0403 +/- 0.0251 IU/ml), and then progressively decreased from the fourth to the seventh days. The t-PA antigen level in aqueous humor increased on the first (0.366 +/- 0.108 ng/ml) and second (0.403 +/- 0.251 ng/ml) postoperative days and gradually decreased from the fourth to seventh days. Under the intracameral condition of the fibrinolytic system, various factors, e.g., serious inflammation or events affecting the balance of coagulation and fibrinolysis, may induce the decrease or depletion of t-PA activity, followed by the pupillary fibrin membrane formation. We suggest that fluctuations of t-PA activity in aqueous humor may affect fibrinous membrane formation over the IOL surface.