The structural stability of bacteriorhodopsin (bR) in purple membrane (PM) and the effects of monovalent anions on the secondary structure of bR at the air-water interface were examined using a monolayer technique and Fourier transform IR spectrometry. The pi-A isotherms of PM monolayer films were condensed in the presence of 50 mM sodium halides. The order of increasing effect was I- > Br- > Cl- > F-. The alpha helical bR molecule in the PM film at the interface is partially transformed into a beta sheet with incubation time after spreading of the PM film. Compared with the alpha helix content of 62% for bR prepared from the subphase water (no ion), the contents in the presence of these ions were 74.4% for fluoride, 78.5% for chloride, 80.4% for bromide and 83.8% for iodide. The addition of anions to the subphase significantly protected the conformational change of bR at the interface. This is due to the direct binding of the anions to the bR molecule in the PM.