The effect of surface adsorption on protein structure is of increasing interest in the biological, medical, and industrial fields. The structure of fibrinogen, a major plasma protein, adsorbed onto TiO2 and HA was examined by employing differential scanning calorimetry (DSC), circular dichroism (CD), and fluorescence spectroscopy. It was found that the TiO2 surface slightly decreased the ordered secondary structure of the protein; the fibrinogen conformation further changed upo washing and desorption. The alpha-helix content decreased gradually during the adsorption, washing, and desorption processes. The results were also used to estimate the overall structure of the protein in the adsorbed and desorbed states. It is significant that the fibrinogen transition enthalpy increased in each case upon adsorption onto HA. The transition enthalpy increased upon adsorption onto TiO2 in two out of three cases tested. These results, combined with the DSC thermograms of fibrinogen at different ionic strengths, suggest that electrostatic interactions are the main mechanism controlling the adsorption of fibrinogen to TiO2 and HA.