The possible role of the LC2 light chain of myosin in the contraction of vertebrate striated muscle has long been a subject of interest. This problem has been addressed in the present study in which the mechanical effects of partial removal of LC2 from skinned fibers of rabbit psoas muscle have been investigated. Each fiber was divided into three segments, thus allowing determinations of the LC2 content of the fiber 1) prior to extraction of the LC2 subunit, i.e. control, 2) following extraction of LC2, and 3) following readdition of LC2 to the fiber. Measurements of isometric tension and the maximum velocity of shortening were made in these fiber segments at each of the above stages of the extraction protocol. LC2 was partially removed from the fiber segments by treatment with a solution containing 20 mM EDTA, 50 mM KCl, 5 mM phosphate buffer, pH 7.0, for 120 min at 30 degrees C, a procedure modified from Chantler and Szent-Gyorgyi (Chantler, P. D., and Szent-Gyorgyi, A. G. (1980) J. Mol. Biol. 138, 473-492). LC2 content was determined using sodium dodecyl sulfate-polyacrylamide gel electrophoretic techniques. The results indicate that removal of about one-third of the total LC2 within a fiber segment reduced Vmax by nearly 50%, with very little effect upon isometric tension. Readdition of LC2 to these fiber segments resulted in recovery of Vmax to near control values. These findings suggest that LC2 may modulate the kinetics of interaction of myosin with actin in mammalian skeletal muscle.