A micropipette suction method was adapted for the characterization of cell adhesion to polymer surfaces. The instrument was applied in experiments probing the adhesion of human erythrocytes to polymer films which had been precoated with monoclonal antibodies against two different transmembrane proteins of the cells. Cells were impinged on the polymer substrates and subsequently removed by stepwise micropipette aspiration. Variations of shape and contact area of the cells during micropipette aspiration-driven detachment were evaluated to determine the separation energy. A strong increase of the separation energy with decreased contact area was observed and explained by the smoothing of the cell membrane at elevated membrane tensions. The results indicate that the overall strength of attachment was determined by the amount and availability of the adsorbed antibodies, while the separation of the cells from the polymer substrates occurred, in general, due to dislocation of the transmembrane proteins from the membrane.