Plasmin cleaves fibrin at or near sites involved in platelet recognition and may modulate platelet adhesion and spreading. Using an in vitro system, we have characterized the effects of limited plasmic degradation of polymerized fibrin on platelet adhesion and spreading. As shown by scanning electron microscopy, exposure to plasmin changed the tight fibrillar fibrin surface to a less dense structure with irregular and broken fibers. There was a gradient of proteolytic degradation through the fibrin clot as shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis with the most extensive degradation at the surface. Plasmic degradation resulted in a rapid and progressive decrease in platelet adhesion. Plasmin exposure for 5 minutes resulted in only 6% solubilization of the fibrin but a 56% decrease in platelet adhesion. After 30 minutes of plasmin exposure, spreading of adherent platelets on fibrin also decreased sharply to a minimum of 35% of baseline. Inhibition experiments with specific monoclonal antibodies (MoAbs) indicated that platelet adhesion to undergraded fibrin involved residues within the sequence 566 through 580 of the alpha chain (including the RGDS site), the carboxyl terminal dodecapeptide of the gamma chain, and the amino terminus of the beta chain. MoAb 7E3, reactive with alpha IIb beta 3, inhibited platelet adhesion to fibrinogen by 90% +/- 5%, and to desA fibrin, prepared with Reptilase (American Diagnostica, Greenwich, CT), by 94% +/- 6%, whereas inhibition of adhesion to undegraded desAB fibrin was significantly less (48% +/- 8%, P > .01). The addition of 7E3 to MoAb T2G1, reactive with beta 15–21, significantly increased inhibition to desAB fibrin to 69% +/- 6% (P < .025), suggesting that the newly exposed amino terminus of the beta chain contributes to platelet adhesion. The results show that plasmin exposure of fibrin markedly decreases platelet adhesion and spreading, suggesting that plasmin degradation may play a role in modulating cellular responses to fibrin.