The filament-to-filament interactions in cardiac alpha-actinin/F-actin mixtures were investigated in the presence of doxorubicin. Stoichiometrical concentrations of the drug in the assembly medium inhibit the growth of alpha-actinin/F-actin three-dimensional structures, as shown by low speed centrifugation, light scattering, A320nm, electron microscopy and low shear viscosity tests. DXR-induced short actin bundle formation could be related to the inhibition of the bundle elongation mechanism, and furthermore, could account for some morphological evidences in DXR-treated living cells. Our results support both the formation pathway of actin bundles as proposed by Stokes and DeRosier (1991), and the observations of Molinari et al. (1990) on the human CG5 cell line.