The subcellular distribution of F-actin, revealed by bound Texas-red-conjugated falloidin, is changed after NAMI-A treatment in HeLa cells, as compared to untreated control cells. A small amount of F-actin cables is found at the edges of control HeLa cells showing accumulation in defined sites. Conversely abundant fibers of F-actin are evident along the entire cell periphery in many NAMI treated HeLa cells . These latter appeared smaller than untreated control cells. Since these cells, observed by phase-contrast microscope, showed the opposite changes in shape (see arrows), the reduced appearance probably reflects a contraction of actin which leaves filopodia and lamellipodia actin-free. These findings on the effect of NAMI-A indicates that this drug can strongly modify the structure of the actin-cytoskeleton, affect integrin function and the adhesion process of HeLa cells to the substrate. These processes are related to cell locomotion and hence with tumour invasiveness.