Langmuir-Blodgett (LB) films of alkaline phosphatase were deposited. The dependence of the enzymatic activity on the conditions of monolayer formation and film transfer onto a solid support was studied. Alkaline phosphatase molecules are very hydrophilic and it is impossible to obtain a significant amount of protein at the surface of pure water. To form a stable monolayer, aqueous solutions of high ionic strength were used. High surface pressures were obtained when 3 M solutions of NaCl or (NH4)(2)SO4 were used as the subphase. The protein films deposited under such conditions possess an enzymatic activity which increases with deposition pressure. The films were characterized by circular dichroism (CD) spectroscopy, ellipsometry and microgravimetry techniques. Alkaline phosphatase molecules in the film are more thermostable than those in solution, The CD spectra of the films remain unchanged up to 200 degrees C, The activity of the protein is retained after heating up to 80 degrees C.