A four-step preclotting method is presented for use with porous filamentous Dacron® prostheses in the fully-hcparinized patient. The method employs controlled fibrin formation within graft interstices, heparin neutralization of all thrombin remaining in the graft wall, and delay of systemic heparin neutralization until 15–20 minutes after clamp release. The resulting flow surface is impervious, smooth and hypothrombogenic. Experimental data are presented which support the rationale of this four-step preclotting method. Four years of clinical experience with the method arc summarized, involving 300 prosthesis limbs used in aortic bifurcation, aortofemoral, femorofemoral, axillary-femoral and femoropoplitcal positions in 192 patients. A clinical perspective of preclotting techniques is presented in which the proper use of this new method is suggested.