A combined scanning probe microscopy study of citrate adsorbed on Au(111) has been conducted. The adlayer structure of citrate on Au(111) is imaged using in situ scanning tunneling microscopy for the first time. Citrate anions are found to form stable and well-ordered adlayers with (4 x 2root3) symmetry. The effect of the citrate adlayer on the interaction between the gold surface and fibrinogen, the protein that has a major role in blood coagulation, has been investigated by atomic force microscopy. The results indicate that citrate modification may change the biocompatibility of gold.