By means of liquid chromatography consisting of gel filtration, anionic exchange, and C8 reverse-phase HPLC, a selective inhibitor of collagen-induced platelet aggregation, named crovidisin, has been purified to homogeneity from the venom of Crotalus viridis snake. Crovidisin is a single-chain, 53-kDa protein with a selective inhibitory activity on collagen-induced aggregation of human washed platelets without affecting those elicited by thrombin, sodium arachidonate, and ADP. Partial sequencing of tryptic digests of crovidisin reveals that partial sequence of crovidisin appears to be identical to that of catrocollastatin, a collagen antagonist occurring in the venom of Crotalus atrox snake. Crovidisin dose-dependently blocked aggregation of human washed platelets triggered by 5 and 10 microg/ml of collagen with IC50 of 0.17 and 0.47 microM, respectively. Not only platelet aggregation but also release reaction, thromboxane formation, and increase of intracellular Ca2+ level of platelets in response to collagen were all completely abolished by crovidisin. In the presence of crovidisin, the Mg2+-dependent adhesion of platelets to collagen was diminished in a dose-dependent manner, while the glycoprotein IIb/IIIa-mediated platelet-fibrinogen interaction was unaffected. When collagen was pretreated with crovidisin and followed by three washes with phosphate-buffered saline, the antiadhesion activity of crovidisin was unaffected. In addition, collagen fibers emitted fluorescence after incubation with fluorescein isothiocyanate-conjugated crovidisin, indicating that crovidisin binds directly to collagen fibers. In conclusion, crovidisin blocks the interaction between platelets and collagen fibers through its binding to collagen fibers, resulting in the blockade of collagen-mediated platelet functions such as adhesion, release reaction, thromboxane formation, and aggregation.