Previously, we reported that the glycosaminoglycan (GAG) hyaluronic acid (HA) specifically bound to the plasma protein fibrinogen [LeBoeuf, R. D., Raja, R. R., Fuller, G. M., & Weigel, P. H. (1986) J. Biol. Chem. 261, 12586]. The binding of other macromolecules to fibrinogen could influence the conversion of fibrinogen to fibrin. Therefore, we tested whether HA and other GAGs could alter the kinetics of fibrin polymer formation and the physical structure of the resulting gel. In this study, we present data showing that the GAGs HA and chondroitin sulfate (CS) affect fibrin formation in three specific ways: (i) they decreased the clotting time of fibrinogen 3-10-fold; (ii) both GAGs increase significantly the rate of fibrin polymer formation; and (iii) fibrin gels containing HA or CS had a final A450 that was greater than controls, indicating that these two glycosaminoglycans influence either the final size of fibrin fibrils or the extent of the lateral association between fibrils. These results demonstrate that the interactions of HA and CS with forming fibrin polymers can alter both the kinetics of formation and may produce structural changes in fibrin gels.