The purpose of this study was to investigate and compare interfacial rheological and tension properties of adsorbed protein films under conditions known to affect bulk properties of proteins. In previous publications, the effects of pH, protein concentration, temperature, and aging on the interfacial rheology of bovine serum albumin (BSA) and human immunoglobulin G (HI(g)G) were reported (D. J. Burgess, L. Longo, and J. K. Yoon, J. Parenteral Sci. Technol. 45, 239 (1991); and D. J. Burgess, J. K. Yoon, and N. O. Sahin, J. Parenteral Sci. Technol. 46, 150 (1992)). These data are compared with interfacial tension data reported here. In addition, the effects of ionic strength and chemical agents on the interfacial rheology and tension of BSA and HI(g)G are reported. An oscillatory interfacial shear rheometer was used to determine interfacial rheology, and a Cahn microbalance connected to a Wilhelmy plate was used to measure interfacial tension. These two techniques provide information on molecular interfacial adsorption, interaction between adsorbed molecules, film compactness, and strength. They appear to be complementary when used in the characterization of adsorbed protein interfacial films.