The effect of ATP on ability of alpha-actinin to increase viscosity of F-actin was measured in three different solutions: 100 mM KCl; 100 mM KCl/l mM Mg2+; and Mg2+ alone at concentrations of 1-6 mM. When ATP and Mg2+ are added at equimolar ratios or at added [ATP] to added [Mg2+] greater than equimolar, alpha-actinin has no effect on F-actin viscosity in the absence of KCl. ATP decreases viscosity of alpha-actinin/F-actin mixtures by 20% even in the presence of KCl, evidently because ATP affects the alpha-actinin-F-actin interaction. Molar ratios of 1 alpha-actinin to 49 actins increase specific viscosity of F-actin approx. 2-fold at 37 degrees C in the presence of 1 mM ATP, so ATP does not prevent the alpha-actinin-F-actin interaction.