The Surface Forces App. (SFA) was used to quant. det. the mol. interactions between layers of protein (hen egg-white lysozyme, Lz) and a thin hydrophobic polysulfone film formed by spin-coating techniques. The normalized intermol. interactions between layers of polysulfone and lysozyme were measured below, at and above the pI of lysozyme (pI = 10.8). Filtration studies using polysulfone membranes were also performed at the same conditions. Adsorption isotherms of the lysozyme on mica and polysulfone film substrates were also measured. The activity of the adsorbed protein was detd. using a fluorimetric assay. The results of this study have direct relevance to a better understanding of the mechanism of protein interactions with polymeric films including membranes, films, tubes and beads and should help in a priori design of new polymeric materials that exhibit low nonspecific protein adsorption.