An in vitro circulation model comprising immobilised human fibrin clots in polyvinyl tubing connected to a peristaltic pump is described in detail. The binding to human fibrin of an 125I-labelled anti-human fibrin monoclonal antibody (mab) 12B3.B10 was studied in sheep plasma. It was found that the accretion of sheep fibrin onto the human fibrin clot did not significantly alter the extent of mab binding to human fibrin in the model. Binding to fibrin in the presence of circulating buffer and human plasma was also studied. It was found that mab binding to human fibrin was significantly less in the presence of circulating human plasma than in buffer or in sheep plasma. In an enzyme linked immunosorbent assay, the binding profiles of mab 12B3.B10 to human fibrin were similar in freshly prepared fibrinogen solutions and buffer, but differed, showing displacement in a fibrinogen solutuon that had been passed through the circulation system. Similar displacement occurred in another freshly prepared fibrinogen solution from a different commercial source. Further, fibrinogen alteration was detected after the addition of the thrombin inhibitor, hirudin, to the circulation fluid, and also despite the omission of the clot from the system. It was concluded that a combination of the action of the peristaltic pump and contact with the polyvinyl tubing caused alteration of the human fibrinogen so that it could displace mab 12B3.B10 binding to the fibrin.