Previous studies have shown that a heterozygous mutation in the fibrinogen Aalpha chain gene, which results in an Aalpha R16C substitution, causes fibrinolytic resistance in the fibrin clot. This mutation prevents thrombin cleavage of fibrinopeptide A from mutant Aalpha R16C chains, but not from wild-type Aalpha chains. However, the mechanism underlying the fibrinolytic resistance is unclear. Therefore, this study investigated the biophysical properties of the mutant fibrin that contribute to fibrinolytic resistance. Fibrin clots made from the mutant fibrinogen incorporated molecules containing fibrinopeptide A into the polymerised clot, which resulted in a 'spiky' clot ultrastructure with barbed fibrin strands. The clots were less stiff than normal fibrin and were cross-linked slower by activated FXIII, but had an increased average fiber diameter, were more dense, had smaller pores and were less permeable. Protein sequencing showed that unclottable fibrinogen remaining in the supernatant consisted entirely of homodimeric Aalpha R16C fibrinogen, whereas both cleaved wild-type alpha chains and uncleaved Aalpha R16C chains were in the fibrin clot. Therefore, fibrinolytic resistance of the mutant clots is probably a result of altered clot ultrastructure caused by the incorporation of fibrin molecules containing fibrinopeptide A, resulting in larger diameter fibers and decreased permeability to fibrinolytic enzymes.