The available surface equation of state for pure pulmonary surfactant monolayers is generalised to binary mixture monolayers by introducing a group of parameters, beta(i) (i=1-4) in the form of x(r)(beta i) to express the influence of the components in new mixing rules and a new factor, I [=(Pi(4)(i=1)beta(i))(1/4)], is defined to represent the interaction intensity between two different components. The pi - A isotherms getting by the surface equations of state agree with the experimental data for protein-lipid binary monolayers, and the average deviation is about 11.41%. The result shows the order of the interaction intensity between protein and lipid is SP-C/DPPG>SP-B/DPPG>SP-C/DPPC>SP-B/DPPC.