Rabbit muscle actin reacts with 2,4-dinitrophenylglutathionyldisulfide, forming a mixed disulfide in position 374. the product S-(cysteine-374)glutathionyl actin forms filaments which are easily disrupted under shearing stress. Even weak mechanical strain, as exerted, for example, during capillary viscometry or heating the solution to 37 degrees C, leads to considerable breakage of these filaments. Because of spontaneous repair which consumes ATP, the mechanically broken filaments exhibit an approx. 6-fold enhanced steady-state ATPase activity as compared to normal F-actin. Monomers of glutathionyl actin have a reduced affinity for their bound nucleotide and a slightly increased critical concentration. Disruption of the filaments and enhanced ATPase activity are reversed by the addition of KCl or the mushroom toxin phalloidin. By the large stabilizing effects of KCl and phalloidin on glutathionyl actin filaments we propose glutathionyl actin as a tool for detecting filament-stabilizing agents and for studying the different mechanisms of filament stabilization.