1. The fluorescence changes accompanying the binding of ATP and adenosine 5′-[gamma-thio]triphosphate (ATP gamma S) to myosin subfragment 1 (S1) and actomyosin subfragment 1 (actoS1) have been reinvestigated at 20 degrees C and 1 degree C, pH 7.0, 0.1 M-KCl. 2. Two successive fluorescence enhancements are observed following ATP binding to both S1 and actoS1. 3. The slow fluorescence change has the same rate with S1 and actoS1, and is due to the ATP cleavage step. 4. With actoS1 the fast fluorescence change occurs after dissociation, so a new intermediate, S1 ATP, is required on the actoS1 pathway. 5. The dissociation of actoS1 by ATP gamma S results in a fluorescence enhancement with the same apparent rate as dissociation, but indirect evidence suggests that this too occurs on a dissociated state.
