The hemostatic efficacy of collagen fibers as a fleece hemostatic agent (CFH) is compared with the efficacy of three commercially available agents: collagen powder (Avitene), absorbable gelatin sponge, and oxidized cellulose. Identical quantities of the four materials were applied to identical multiple spleen incisions in five dogs; CFH and collagen powder proved three times as effective as the other two materials. CFH and collagen powder were then compared with regard to hemostatic efficacy, handling characteristics, and tissue reaction. Different weights of the materials were applied to multiple spleen lacerations in 12 dogs and spleen avulsions in 12 dogs. The hemostatic efficacy of the materials was similar, but CFH was superior with regard to handling characteristics and tissue reaction. Another experiment, with six pigs, showed that CFH can be successfully sutured as a patch to profusely bleeding lacerations of the spleen, liver, and kidney.