The combined effects of ATP concentration and ionic strength were studied in an actomyosin in vitro motility assay using skeletal and cardiac myosin. The velocity of actin filaments increased up to a critical ionic strength, at which filament sliding stopped. At or above the critical ionic strength, filaments did not slide, but wiggled while focally attached to the surface. At these high ionic strengths, when the ATP concentration (originally 1 mM) was progressively reduced (down to submicromolar levels) by rigor-solution washes, the stationary, wiggling actin filaments promptly started to slide. The effect was reversible; upon adding ATP again, the sliding movement stopped, and wiggling began. The ATP washout-induced motility at high ionic strength may be explained by an electrostatic mechanism which determines the affinity of myosin to actin. The critical ionic strength was different for skeletal and cardiac myosin. For skeletal it was 77 mM, while for cardiac it was only 57 mM. Cardiac myosin's lower critical ionic strength implies a lower affinity to actin.