Single filaments of actin were observed to fluctuate in the direction
perpendicular to their longitudinal axes when they hydrolyzed ATP in t
he presence of myosin. The transversal fluctuations of actin filaments
were identified by reading the transversal displacements of the filam
ents under a fluorescence microscope. The transversal fluctuations in
the absence of ATP decreased their intensity as the number of myosin m
olecules contacting directly with actin filaments increased. In the pr
esence of ATP, on the other hand, the amplitude of the transversal flu
ctuations increased in proportion to the ATP concentration up to a cer
tain level, while the sliding velocity of the filaments did not increa
se significantly over the same range of ATP concentration. The present
observation suggests that the chemical energy released from ATP along
actin filament binding to myosin molecules is first and primarily con
verted into kinetic energy of fluctuations in the form of the displace
ment movements of the filaments in the direction perpendicular to thei
r longitudinal axes.