The staphylococcal fibrinogen binding protein of a strain of coagulase-negative Staphylococcus aureus was purified 229 fold in terms of the haemagglutination unit compared to the starting material by affinity chromatography on fibrinogen-Sepharose. By polyacrylamide gel electrophoresis in both reduced and unreduced gels, the protein showed one major band and minor bands with relative molecular masses of 62,000, 61,000, and 59,000, respectively. The isoelectric point was between 10.2 and 10.8 determined by isoelectric focusing in polyacrylamide gel. By the agar diffusion test one band was obtained against anti-whole cell rabbit serum. Amino acid analysis of fibrinogen binding protein showed glycine, glutamic acid, lysine, alanine, aspartic acid, and arginine as the major components. Protein A or teichoic acid extracted from the homologous strain did not show fibrinogen binding activity assayed by haemagglutination and anti-fibrinogen binding activity of anti-whole cell Fab. The amount of the fibrinogen binding protein to absorb the activity of anti-whole cell Fab was decreased to 1/60 of that of the starting material. Sheep red blood cells coated with the fibrinogen binding protein agglutinated in 0.001% (w/v) fibrinogen solution.