Current inotropic therapies used to increase cardiac contractility of the failing heart center on increasing the amount of calcium available for contraction, but their long-term use is associated with increased mortality due to fatal arrhythmias. Thus, there is a need to develop and explore novel inotropic therapies that can act via calcium-independent mechanisms. The purpose of this study was to determine whether fast -myosin molecular motor gene transfer can confer calcium-independent positive inotropy in slow -myosin-dominant rabbit and human failing ventricular myocytes. To this end, we generated a recombinant adenovirus (AdMYH6) to deliver the full-length human -myosin gene to adult rabbit and human cardiac myocytes in vitro. Fast -myosin motor expression was determined by Western blotting and immunocytochemical analysis and confocal imaging. In experiments using electrically stimulated myocytes from ischemic failing hearts, AdMYH6 increased the contractile amplitude of failing human [23.9±7.8 nm (n=10) vs. AdMYH6 amplitude 78.4±16.5 nm (n=6)] and rabbit myocytes. The intracellular calcium transient amplitude was not altered. Control experiments included the use of a green fluorescent protein or a -myosin heavy chain adenovirus. Our data provide evidence for a novel form of calcium-independent positive inotropy in failing cardiac myocytes by fast -myosin motor protein gene transfer.—Herron, T. J., Devaney, E., Mundada, L., Arden, E., Day, S., Guerrero-Serna, G., Turner, I., Westfall, M., Metzger, J. M. Ca²⁺-independent positive molecular inotropy for failing rabbit and human cardiac muscle by -myosin motor gene transfer.