In an cell aspiration experiment, a cell is partially sucked in a micropipette by applying a suction pressure. If the applied pressure is larger than a threshold - which depends sensitive on the cell type - the cell membrane detaches from the underlying cortex, leading to the formation of a bleb that protrudes into the pipette. If a good model relating pressure and cell deformation is available, the adhesive strength of the cortex-membrane linkers can be calculated. We have implemented a model for the cell deformation that treats the cortex layer as a viscoelastic active gel. The model appears to give a behaviour similar to that found in experiments and enables to differentiate between different types of cells based on their mechanical response.