The interferometric surface force technique has been employed to determine how the structural stability of globular proteins affects their adsorption and the interactions between adsorbed protein layers. The system consisted of positively charged bacteriophage T4 lysozyme and negatively charged mica surfaces. The wild type and one synthetic mutant of the protein, Ile3 --> Trp, differing in structural stability while the total charge and tertiary structure are the same, were studied. The adsorption leads to a nearly complete neutralization of the negative surface charge of mica, reducing the long-range electrostatic double-layer interaction acting between mica surfaces. The thickness of the adsorbed layer is for the wild type consistent with the dimensions of the protein, while the Ile3 --> Trp mutant gives a layer with a thickness smaller than any of its native dimensions. Another consequence of the difference in structural stability is that the short range attraction between one protein layer and one bare mica surface is an order of magnitude larger for the Ile3 --> Trp mutant than for the wild type. The results demonstrate that the less stable mutant loses its tertiary structure upon adsorption, whereas the wild type retains its globular shape. These differences provide an understanding for the differences in adsorbed amount and complements the information about changes in secondary structure upon adsorption observed with other methods.