We studied the effects of PRAP-1, a PAI-1-inhibiting polyclonal antibody, on tissue-type plasminogen activator (t-PA)-induced lysis of arterial-like thrombi prepared in vitro from rat and human blood in a Chandler loop device. The t-PA induced lysis of the thrombus head and tail was studied over 5 h. For thrombi prepared from rat blood, the lysis ratio head:tail was (mean × SD): 0.71 × 0.16 (platelet-poor blood), 0.49 × 0.22 (whole blood), and 0.22 × 0.12 (platelet-rich blood). PRAP-1 increased the lysis ratio to 0.68 × 0.22 for whole-blood thrombi (P < 0.01) and to 0.46 × 0.15 for platelet-rich thrombi (P < 0.001). For thrombi formed from human whole blood, PRAP-1 increased the lysis ratio head:tail from 0.61 × 0.18 to 0.95 × 0.24 (P < 0.05). These effects of PRAP-1 were due to a selective increase in the lysis of the thrombi head. The plasminogen activator inhibitor (PAI-1) activity was 7.5 × 2.8 (rat) and 3.4 × 1.3 (human) times higher in the thrombus head compared with the tail. In thrombi prepared from rat whole blood, PRAP-1 decreased the PAI-l activity of the thrombus head only by 38 × 7% (P < 0.01), while in thrombi prepared from human whole blood the PAI-1 activity of the thrombus head decreased by 83 × 4% (P < 0.001). In conclusion, the lysis resistance of the head of an arterial-like thrombus formed in vitro can be partially (rat) or totally (human) normalized by inhibition of the PAI-1 activity. The results further suggest that in rat arterial-like thrombi a t-PA-inhibiting component, different from PAI-1, is present. Blood Coag Fibrinol 9:11–18 × 1998 Rapid Science Ltd.