In the present study, Fourier transform infrared-attenuated total reflection spectroscopy has been used to characterize Langmuir-Blodgett films of purple membranes deposited on Ge crystals at different surface pressures. The results obtained indicate that the average angle of the protein’s cr-helices with respect to the interface normal remains unchanged with increasing lateral pressure. Moreover, the total absorbance of the films as a function of molecular area suggests that a multilayer structure is formed between 40 and 46 mN m(-1).