Atomic force microscopy has been used to investigate the adsorption of the plasma protein fibrinogen on graphite and mica substrates. These substrates serve as model hydrophobic and hydrophilic surfaces, respectively. The overall structure of submonolayer coverage films is dramatically different on the mica and graphite substrates when imaged under ambient conditions after being dried under a nitrogen stream. The molecules show a tendency to aggregate on the graphite substrate but adsorb as isolated single molecules in the case of mica. On the mica substrate, individual fibrinogen molecules appear globular in structure whereas, on graphite, the trinodular structure is most commonly observed. The average height of the fibrinogen molecules as measured by tapping mode AFM in air is 1.71 ( 0.65 nm, and the average height on the graphite substrate is 1.05 ( 0.13 nm. The average lengths and widths of the molecules on these two substrates vary as well, the average length being 31 ( 7 nm on mica and 63 ( 10 nm on graphite. These differences are consistent with a change in conformation of the protein upon adsorption of these two surfaces due to the differences in surface chemistry of the substrates, which suggests a change in mechanism of adsorption between the two substrates.