Joint work with Igor R. Kuznetsov and Marc Herant. The transport of labeled G-actin from the mid-lamella region to the leading edge in a highly motile malignant rat fibroblast line has recently been studied using fluorescence localization after photo bleaching or FLAP (see Zicha et al.[Zicha2003]). The transit times recorded in these experiments were so fast, that simple diffusion was deemed an insufficient explanation. Since this conclusion has been controversial we here we re-examine the Zicha-FLAP experiments using a two-phase reactive interpenetrating flow formalism to model the cytoplasm and the transport dynamics of bleached and unbleached actin in a moving cell. This new analysis reveals a mechanism that can realistically explain the timing and the amplitude of all the observed FLAP signals in the Zicha-experiments without invoking special transport modalities. The proposed mechanism requires the existence of a small compartment at the leading edge of the lamella where actin polymerization is very fast and where this production is balanced by equally fast mechanical dilatation of F-actin caused by retrograde flow away from the leading edge. If our dilatation hypothesis is correct, the FLAP technique constitutes a novel and very sensitive probe of actin dynamics in a crucial leading edge environment which is otherwise very difficult to interrogate.