Adrenocorticotropin hormone (ACTH) is able to induce motile events in the phagocytic hemocytes of Viviparus ater by modifying the cytoskeletal components. The cell shape changes into a polarized morphology. The microfilament bundles, which, in control cells display a radial distribution from the nucleus to the cell periphery, are arranged under the plasma membrane. Moreover, on the protruded lamellipod, actin is accumulated in small, round structures. The microtubule component increases and seems to contribute to the maintenance of cell polarity. Hemocyte adhesive properties are also modified, and different localization patterns of extracellular fibronectin are observed. Cellular responses induced by the ACTH signal may be mediated by cyclic 3',5'-AMP (cAMP).