Using Langmuir-Blodgett and fluorescence microscopic techniques, the interaction between staphylococcal protein A (SpA) and dipalmitoylphosphatidic acid (DPPA) was characterized on water and substrate surfaces. The conformation of SpA in the monolayer was also determined. Results show that the distribution of SpA in the deposited monolayer is strongly influenced by the phase transition of the monolayer, and the activity of the protein in the monolayer depends on the surface pressure, especially in the plateau range.