A series of analyses were performed on fibrin-based adhesives to describe their failure characteristics. Two test methods were used: uniaxial, monotonic tensile testing of the bulk material, and blister testing using fresh porcine-source skin graft as the adherend. Two fibrin concentrations, high (HFC), and low (LFC), were used to investigate the effects of the gel matrix density upon mechanical properties. In tensile tests, fibrin gels strain hardened, as functions of percent strain and of strain rate. An increase in modulus of elasticity (E) was seen with increasing strain and strain rate at both tested fibrin concentrations. Mode I failure mechanisms were predominant. Both adhesives appeared to fracture from the outer edge to the interior of the specimen at slower strain rate tests. This trend reversed as strain rate increased, becoming a classic "cup and cone" ductile fracture. Syneresis occurred at both concentrations at lower strain rates, but was more pronounced for the LFC. Ultimate tensile strength and E were greater for the HFC than for the LFC at all strain rates, decreasing with increasing strain rate. In the blister test, the failure locus changed from cohesive to adhesive as the strain rate was increased for the HFC. Failure of fibrin gels likely occurs by percolation of the pressurized saline, displacing the entrapped liquid phase of the gel in regions of relatively low moduli and strength, leading to fracture of the matrix. For LFC, the overall fracture locus remained predominantly cohesive regardless of strain rate. Burst strength and failure energy were higher for HFC than for LFC. It would appear that fibrin acts more as a viscous liquid than a rubberlike/elastic material at lower concentrations because adhesive failures had a higher burst strength and fracture energy (Gc) than did cohesive failures. © 2001 John Wiley & Sons, Inc.