The main task of leukocytes is to patrol throughout the body in order to detect and eliminate potentially harmful structures such as invading microorganisms or effete cells. The fulfilment of these functions often requires that cells undergo drastic morphological changes. These changes may be passive, e.g. when a spherical neutrophil flowing in blood acquires an elongated shape in order to pass through a small capillary vessel, or they may be active, when a resting cell sends forward a thin lamellipodium and starts migrating on a surface (Stossel, 1993). It is usually considered that cell shape is essentially controlled by a complex cytoplasmic network including at least three structural species: microfilaments, microtubules and intermediate filaments (see the review by Richelme et al., 1996, for more information). Extensive interactions between these structures were described, and all of them may contribute cell morphological and mechanical properties. However, for the sake of simplicity, we shall focus on actin microfilaments. Indeed, actin represents more than 10% of leukocyte cytosolic proteins, and much experimental evidence demonstrated that microfilament alteration strongly influenced cell mechanical properties.