The membrane stiffness (epsilon) of rat lung fibroblasts (RLFs) adhered on different polymeric surfaces was probed by atomic force microscopy. The corresponding cell morphology was also analyzed to probe its interrelationship with epsilon. Two tyrosine-derived polymer families, poly(DTR glutarate)s and poly(DTE-co-PEG(1000) carbonate)s with systematic variations in the chemical composition and physical properties, notably surface hydrophilicity, were used. The cell membrane of adhered RLFs was indented by a probe tip. epsilon was obtained by best-fitting the relationship of applied tip forces and the indentation depth with the Hertz model. Excluding tissue culture polystyrene, non-PEG-containing polymers are generally hydrophobic and the changes in chemical composition do not elicit significant changes in epsilon. In contrast, polymers containing as little as 2mol.% PEG display a major increase in surface hydrophilicity and invoke a substantial decrease in epsilon. Additionally, RLFs show a high degree of spreading and fibroblastic appearance on non-PEG-containing polymers, but much less spreading and axial morphology when PEG is present. A mechanism is proposed to explain how a cell maintains its structural integrity on different polymeric surfaces: the degree of cell spreading is higher on non-PEG-containing surfaces than on PEG-containing ones, resulting in more extended cytoskeletal filaments and hence a stiffer cell membrane. Our studies shed light on the use of cellular micromechanics, and in particular membrane stiffness, to characterize cell response as a function of the chemical composition of the underlying substrata.