A biomaterial with a thromboresistant surface and a method for forming same are provided. The thromboresistant surface is comprised of a distinct coating layer of a chitosan-based membrane and a biologically active material. The biologically active material is capable of converting the chitosan membrane coating from a highly thrombogenic to an essentially non-thrombogenic one. The biologically active material can be a polymeric substance, such as polyvinyl alcohol, forming a polymeric blend with the chitosan, or, can be a biological substance, such as serum albumin, embedded in or attached to the chitosan membrane which has been activated with a treatment of glutardialdehyde. The thromboresistant biomaterial is suitable for use in vascular grafts having an inside diameter of less than 6 millimeters.